DIRECT INJECTION

The following is a technical piece on direct Injection, two-strokes and other topics submitted to Dirt Bike by Michael Traum.
In the September 2016 issue of Dirt Bike Magazine the “Work-Horse”
story about the new 2017 KTM/Husqvarna 300 two strokes was an
interesting read that made me want to get out and ride.  Mostly the
story was of interest to me because I just got my ’86 Husqvarna 400 WR
fixed up with a bunch of nice parts off of a 1986 Husqvarna 250 WR,
including a stiffer than stock shock spring which is much more
appropriate for my 200 pound plus weight and aggressive riding
tendencies.  All it seems to need now is a set of 40mm upside down
White Power forks off of a 1991 Husqvarna WMX 610.  I actually do have
an extra set of 1991 WMX 610 forks I got from a dismantler in Oregon
along with the front wheel, caliper and triple clamps, but I have been
reluctant to make the upgrade.  As it is the abrupt two stroke power,
less than precise 40mm conventional forks, single piston Brembo front
caliper and the drum rear brake sort of seem to all go together.  It
is just not as fast as a modern four stroke even though it makes tons
of power.  As long as it is kept in mind that it won’t go quite as
fast it is a fun bike.  The big power is a blast, and the suspension
is supper plush and comfortable.  It’s just not much of a race bike,
and that has to be kept in mind.  The flexi 40mm conventional forks
are of course the biggest hindrance to going fast, but the drum rear
brake really is a lot slower also .  The drum rear brake appears to
work just fine, but modulation when braking hard to setup for a turn
is not as good as a disk rear brake.  I guess it is just that I have
gotten so used to riding with a disk rear brake that I can’t do
without it.  With the flexi 40mm conventional forks the drum brake is
much less of a problem, the flexi forks are a near constant reminder
that the bike is not all that fast.  Slow down and smell the two
stroke smoke one might say.
 
The reason I am writing in is to complain about the introduction to
the September 2016 Dirt Bike Magazine article on the 2017 Husqvarna
TX300.  On page 32 you write:  “Why no injection?  Because it still
wouldn’t pass EPA or CARB emission tests as an off-road vehicle.  The
only two-strokes capable of that are massive direct-injection designs
that require extreme fuel pressure, and that would defeat the whole
point of having a two-stroke in the first place.”  This is sort of
true, but the wording is misleading.
 
The phrase “…massive direct-injection designs that require extreme
fuel pressure…” makes the reader think you are talking about a
diesel engine.  Two stroke diesels and GDI two strokes are very
different.  Direct injection on any gasoline engine can be confusing
because the distinction between a gasoline engine and a diesel engine
gets blurred if the GDI engine is capable of running with time of
combustion injection.  Most existing GDI engines, two stroke or four
stroke, make little or no use of time of combustion injection.  They
are EFI gasoline engines where the injectors are moved from the intake
ports to the combustion chamber.  Moving the injectors to the other
side of the intake valves is advantageous for a variety of reasons.
The big advantage for four stroke engines is that the gasoline does
not block the flow of intake air past the intake valves, allowing for
higher performance over a wider range of elevated engine speeds even
on a radically under square bore and stroke configuration.
 
For a two stroke the really big advantage of direct injection is that
fuel does not have to be blown out the exhaust unburned, resulting in
more efficient and much cleaner operation. Direct injection itself is
however not a total solution for getting two strokes to run clean.
The problem is that the bottom end still needs to be lubricated.  The
solution that outboard motor and snow mobile makers have used is to
blow only a small amount of the gasoline through the crankcase to
deliver the two stroke oil to the bearings and pistons.  This sort of
works.  It does dramatically reduce the amount of gasoline blown out
the exhaust unburned, but lots of lube oil is still blown out the
exhaust unburned.  Electronic controls to meter the amount of lube oil
injected into the intake air based on engine speed and load offer an
additional reduction in emissions, but lube oil is still blown out the
exhaust.
 
The only way to really clean up a two stroke is to remove the case
reed induction.  Without case reed induction a pressure lubrication
system can be used to lubricate the bearings and pistons and no two
stroke oil is required.  The main problem would appear to be in
starting a two stroke without case reed induction.  Some small two
stroke motors do in fact have the intake port running directly into
the cylinder wall without case reed induction.  It obviously can be
made to work, at least on a small engine that can be easily spun up to
high speeds.  Is that why an electric start 2017 two stroke is so
significant?  Perhaps.  A starting blower of some kind is also a
possibility, and somewhat better low end power would be possible with
the intake air being blown in.  An electric motor driven centrifical
starting blower would seem to be the best option for a light weight
and compact two stroke dirt bike engine.  Up in the power band the
electric motor driven blower would not need to run, and a bypass valve
would allow unrestricted flow from the air box to the intake.  Yes
that all adds weight and complexity, but not “massive direct-injection
designs that require extreme fuel pressure”.
 
A clean two stroke that is still light weight and compact would be
possible, but it certainly takes some doing.  Even without electronic
controls a clean GDI two stroke would still be possible.  The
injection system is not much of a problem as only one parameter needs
to be modulated. When the gasoline is injected before combustion takes
place the rate of injection is not critical.  Any old mechanical
injection system would work fine, and since the pressures required are
quite low the pump could be made much lighter and more compact than
what is required for diesel engines.  The main obstacle is just the
starting blower.  Using an engine driven roots blower like the Detroit
(GM, Jimmy) two stroke diesels did is an option, but not necessarily a
good option.  The roots blower delivers more pressure at higher rotor
speeds.  That is much better for a forced induction four stroke
gasoline engine than for starting a two stroke.  A clutch driven roots
type starting blower with a bypass valve would be another option.  Or
the same thing could be done with an electric motor driven centrifical
starting blower on a simple switch.
 
In any case a clean running GDI two stroke is a very different engine
than a carbureted case reed induction two stroke.  A clean running GDI
two stroke can however be a lot more like a carbureted two stroke than
“…massive direct-injection designs that require extreme fuel
pressure…”
 
But what about powering a dirt bike with a diesel engine?  It
certainly would be possible, especially if broad low end torque is
desired over screaming huge top end pull.  Page 32 of the September
issue of Dirt Bike Magazine goes on to say:  “In Europe, there will be
a fuel injected version of this bike, but from what we understand, it
will be dumbed down to less than 20 horsepower.”  A 72mm stroke length
diesel would run best in the 2,000 to 4,000RPM range of engine speeds.
At 4,000RPM a 300cc two stroke diesel would be able to do
approximately 20hp.  The real limitation to performance would be the
injection system.  With a competent injection system capable of
matching the injection flow rate and injection start timing to the
requirements of the engine good operation over a wide range of engine
speeds would be possible. If the injection flow rate could be
sufficiently reduced high efficiency in the 2,000 to 2,500RPM range
would make for spectacularly high fuel mileage at low speeds.  If the
injection flow rate could also be high for higher engine speeds then
even bigger power up to around 5,000RPM would also be possible.  With
a fixed injection flow rate light load efficiency would suffer greatly
and top end performance would be flat and lifeless.  Even with a fixed
injection flow rate a 300cc two stroke diesel dirt bike might sort of
work, but the injection flow rate would need to be set for a
particular type of riding.  Anyone who wanted some power would require
a high injection flow rate for good operation under a heavy load up in
the 3,000 to 4,000RPM range.  If light load efficiency and good fuel
mileage at slow trail speeds were desired then a much lower injection
flow rate would be required.
 
What it comes down to is that with a high fixed injection flow rate a
300cc two stroke diesel would be a lot like a poorly performing
gasoline engine with no top end power.  The difference though is that
even with a fixed injection flow rate a diesel engine can attain
higher efficiency while running on much cheaper fuel.  Because diesel
engines tend to get rather dirty at elevated engine speeds though it
makes a lot of sense to use an injection system that is capable of
reducing the injection flow rate for efficient and clean operation at
lower engine speeds.  A 72mm stroke length diesel has too low of a
mean piston speed to do at all well below about 2,000RPM, but in the
2,000 to 2,500RPM range reasonably good light load efficiency could be
attained.  Under a more substantial medium load the 72mm stroke length
diesel would do best up in the 2,500 to 3,000RPM range of engine
speeds, and again rather good efficiency could be attained with
reasonably low emissions.  The problem with such a short 72mm stroke
length for a diesel engine is that peak efficiency under heavier loads
would be attained up in the 3,000 to 3,800RPM range, which is getting
pretty fast for a diesel engine to be able to run clean.  With a high
injection flow rate big 25hp power could be available to 5,000 or even
5,500RPM, but the exhaust is going to get somewhat blackened up at
those very high engine speeds.
 
A 72mm stroke length is really somewhat too short for a diesel engine,
but that does not mean that it can’t be made to work.  The short 72mm
stroke length would best be used with a competent injection system
that is capable of a high injection flow rate for big power up to at
least 4,000RPM while also being capable of a low injection flow rate
for good light load operation down to 2,000RPM.  As with any cylinder
port two stroke the power band would tend to be narrow, but that could
be made to somewhat work as a diesel engine.  Big power would come
over a rather narrow range of elevated engine speeds, but quite a bit
of overrev out to 5,000 and even 6,000RPM could be available for
getting into the next gear.  At lower engine speeds the cylinder
filling is going to be somewhat low, but that is fine for a diesel
engine running under reduced loads with a reduced injection flow rate.
A power valve could help with broadening the torque down to lower
engine speeds.  Overhead camshaft driven exhaust valves instead of an
exhaust port would be even better, but less compact of course.
 
For the same reasons that carbureted or EFI four stroke dirt bikes
work better than carbureted two stroke dirt bikes it would make a lot
of sense to instead build a four stroke diesel dirt bike engine.  The
longer the stroke the better really because anything less than a four
inch stroke length on a diesel engine just does not accomplish much of
anything.  In the interest of compactness and overall light weight
though going down to as little as three inches of stroke length could
make sense for a four stroke diesel dirt bike.  The range of engine
speeds would be the same 2,000 to 5,000RPM, but a four stroke with a
competent injection system could provide much broader big torque from
2,500 to 4,500RPM while still being able to do just as well down to
2,000RPM under light loads.
 
If really big power from a simple and light weight normally aspirated
four stroke diesel dirt bike was required then getting as much
displacement as possible would be the way to go.  The Husqvarna dirt
bikes from 1960 up to 2004 were obviously capable of handling a three
and a half inch stroke length engine.  Putting a four inch bore on
that three and a half inch stroke length results in about 700cc of
displacement, which would be good for a bit more than 20hp at around
3,500 to 4,000RPM.  Again even more power could be attained up at
5,000RPM, but that high of an engine speed on a diesel engine does
result in dirty exhaust.  Even more significantly the longer three and
a half inch stroke length would allow for some reasonably good light
load efficiency down to around 1,700RPM.  Under a medium load the
engine would still do best above about 2,000RPM and peak efficiency
under a full load would probably come up around 2,400 to 2,800RPM.
 
The interesting thing to keep in mind when considering a diesel
powered dirt bike is that a large single cylinder configuration fits
very nicely.  Because more stroke length is desirable for a diesel
engine a big single cylinder engine gets a lot of displacement into a
small space.  A single cylinder four stroke engine does result in poor
transmission efficiency, but we are already used to that particular
problem.  The poor transmission efficiency on a single cylinder four
stroke only very slightly hurts full load performance.  Rather it is
light load performance that suffers so greatly on a single cylinder
four stroke.  What this means is that a big single cylinder four
stroke diesel dirt bike would work best for riding fairly hard and
fast as opposed to cruising along slowly at very low power output.  A
diesel powered dirt bike that needed to do really well over a range of
lower speeds would probably work out best as a small bore inline two
or inline three cylinder engine.  Such an engine would have a
radically under square configuration, again with the longest stroke
that would fit.  A three and a half inch stroke length two cylinder
engine with two and a quarter inch bores would displace 450cc.  Power
might be around 13hp at 3,500RPM and it would be a bit more difficult
to get more power to 5,000RPM with the radically under square
configuration.  The minimum engine speeds would be roughly the same as
for the 3.5 inch stroke length 700cc single, but the small bore engine
would be cleaner and would have the potential to be slightly more
efficient at medium engine speeds.
 
The big problem with diesel power for dirt bikes is that getting more
power is difficult.  Just 20hp is plenty for getting around, but it’s
pretty slow for racing.  Especially when you are already used to
ridding 40 and 50hp bikes.  The options for increasing power output
are limited since revving a diesel engine higher does not work out
well.  It is possible to get power out of diesel engines up to even
higher engine speeds around 6,000RPM, but they get extremely dirty and
the amount of additional power is just not worth it.  The only things
that can be done to get more power from diesel engines is to make them
bigger or use forced induction.  Even more than 700cc would be
possible with a single cylinder diesel dirt bike engine, but it does
get bigger and heavier.
Turbo charging diesel engines certainly does allow for more power
output, but the turbocharger adds both complexity and weight.  Diesel
engines do very well with forced induction because more air results in
more complete combustion and lower emissions.  To increase power
output by a large amount though the compression ratio should also be
reduced.  There is no absolute maximum cylinder pressure for diesel
engines, but going beyond certain levels of cylinder pressure result
in nastier more toxic emissions.  Reducing the compression ratio down
to 14:1 like some old fashioned diesel engines from the 1930’s used is
a great idea for a high output turbocharged diesel engine, but then
cold starting can become somewhat of a problem.
Possibly the best high output diesel dirt bike engine would be a
single cylinder two stroke with overhead exhaust valves and roots
blower.  Cylinder port two strokes can’t use forced induction because
the intake ports close before the exhaust port.  An overhead valve two
stroke though can use a much earlier exhaust valve closing time to
make use of forced induction.  Overhead exhaust valves not only make
forced induction possible, but they also allow broader torque down to
much lower engine speeds.
A 700cc two stroke diesel with four overhead exhaust valves and a
roots blower could make some very big power, probably at least 60hp at
4,000RPM.  Because there is only a very short period of time when the
exhaust valves are closed and the intake ports are still open the
boost in power output with forced induction tends to be rather small
on a two stroke diesel.  In a way this is good though, as there is
less difficulty with picking a compression ratio.  A moderate
compression ratio works just fine, both starting easily and keeping
nasty high pressure pollutants to a minimum.
Four stroke diesel engines can run more boost for high output despite
half as many power strokes.  A 700cc turbocharged four stroke diesel
could also do 60 or 70hp output at 4,500RPM.
 
The ultimate high output diesel dirt bike engine might be a two stroke
with no cylinder ports.   With both the intake valves and the exhaust
valves in the head variable valve timing systems could be used to keep
the intake valves open later under high output at high engine speeds.
Fully variable valve timing would also mean that the engine could
actually start and run as a four stroke, eliminating the need for any
sort of starting blower.  Imagine an engine that could start, idle and
run under light to medium light loads at low to medium engine speeds
as a four stroke diesel, but then could switch to running as a
turbocharged two stroke diesel to make big torque and big power.
A 700cc single cylinder diesel with all-in-head valves and a
turbocharger might do in excess of 100hp at 4,000 to 4,500RPM.  Since
that is way more power than a dirt bike needs this engine could even
be scaled down somewhat.  A smaller bore under square configuration
makes an all-in-head valve two stroke more difficult to get to flow
well, but if the goal is a smaller size with a lower power output
level then a 400cc engine that makes around 45 or 50 at 3,500 to
4,000RPM would be entirely possible. That 400cc engine might use a
stroke length anywhere from three to three and a half inches.  The
three inch stroke length would be desirable for a lower center of
gravity and an overall more compact engine, but the longer three and a
half inch stroke length would run somewhat better as diesel engine.
Even if the under square 400cc diesel engine had some difficulty
flowing well up at high engine speeds it could still provide good
torque over a reasonable range of engine speeds as the longer stroke
would allow it to run down to somewhat lower engine speeds.  When less
power output is desired under square diesel engines tend to work out
very well.
 
A variable valve timing system and an injection system that can switch
back and forth between two stroke and four stroke operation is
technically feasible, but it does involve quite a lot of computerized
electronic controls.
 
All-in-head valves could be used on a two stroke without fancy
computer controls, but it would use a roots blower instead of a
turbocharger.  With a roots blower the all-in-head valve two stroke
would start as a two stroke and would always run as a two stroke, but
the intake valves could open sooner and close sooner for higher
efficiency at the rather low 2,000 to 3,500RPM of a diesel engine.
 
There seems to be a big advantage to overhead exhaust valves versus an
exhaust port, the intake ports though are much less of a problem.  It
is getting the exhaust valves to close earlier that is so very
important on a two stroke diesel, both because it allows forced
induction and also because earlier closing exhaust valves work so much
better at the lower engine speeds of a diesel engine.  Getting rid of
the exhaust port also dramatically increases the longevity and
reliability of a two stroke.  Exhaust ports get hot and tear up the
compression ring something awful.  Intake ports stay cool and are not
nearly so much of a problem.  For a diesel two stroke the intake ports
stay very low and small, but because they can go all the way around
the cylinder there is still a reasonably large total port area.  If
the intake ports stay in the cylinder walls then there is more room in
the cylinder head for exhaust valves, and more total exhaust valve
area is a big advantage for a two stroke where the valves are open for
such a short length of time.
 
For a dirt bike there is also one other unique consideration that
would tend towards leaving the intake ports in the cylinder walls.
Going with overhead exhaust valves makes the engine taller, which
would favor a shorter three inch stroke length versus a three and a
half inch stroke length.  The shorter three inch stroke length would
work better up at somewhat higher engine speeds than the longer three
and a half inch stroke length.  Up at the slightly higher engine
speeds the intake ports being open the same length of time before and
after bottom dead center is less of a problem.  The more slowly a
diesel engine spins the more undesirable the symmetric intake port
timing becomes.  At slower engine speeds the limited exhaust valve
area with an all-in-head valve two stroke is also less of a problem,
where at higher 4,000 to 5,000RPM engine speeds the larger total
exhaust valve area is a big advantage.
If however the goal is extreme simplicity and fairly big diesel power
from a compact engine then a 700cc normally aspirated cylinder port
two stroke might be just the thing.  At around 40 to 43hp at 3,500 to
4,000RPM it’s enough to be a real dirt bike.  A fairly big dirt bike
with a three and a half inch stroke and four inch bore, but a cylinder
port two stroke does keep the total engine height low even with that
rather monstrous stroke length.  The main challenges remain an
injection system capable of delivering a substantially lower injection
flow rate at reduced engine speeds and some form of starting blower.
Since a cylinder port two stroke really is always a normally aspirated
engine it hardly makes sense to spin a roots blower all the time.  An
electric motor driven starting blower with a bypass valve seems like a
much better idea.
A case reed induction two stroke diesel would also be possible, but it
would not be anywhere near as clean running.  Any type of case reed
induction two stroke has some problems with the two stroke oil going
out the exhaust unburned.  Some of the two stroke oil is just going to
get blown straight out the exhaust totally unburned.  Overhead exhaust
valves and case reed induction seems somewhat appealing for a slow
spinning diesel engine, but it is not actually as good as it might
first seem.  Getting the exhaust valves to close earlier should help
to reduce the amount of two stroke oil blown out the exhaust unburned,
but this is actually a very limited advantage.  The problem is that
two stroke engines always have the intake and exhaust valves open at
the same time, and this results in some of the intake air flowing out
the exhaust.  When the lube oil is mixed with the intake air this
results in some of the lube oil blowing out the exhaust unburned.  The
key to a clean running two stroke remains eliminating the case reed
induction and using a pressure lubricating system.
As far as the injection system goes there seems to be little reason to
use anything other than a basic regulator valve type injection pump.
For a single cylinder engine the regulator valve type injection pump
is extremely simple.  An advance mechanism might be desirable, but a
cylinder port two stroke diesel could easily get away with just a
regulator valve for controlling the fuel delivery.  A regulator valve
type injection pump tends to have a bit of an advancing effect built
right in because the injection start timing becomes somewhat earlier
when larger quantities of fuel are injected.  That small advancing
effect combined with the increase in injection flow rate when larger
quantities of fuel are injected adds up to an injection system that
actually can work reasonably well.
 
A cylinder port two stroke diesel which has lower cylinder filling at
the lower engine speeds is a particularly good match for a simple
regulator valve type injection pump.  Lower power output at lower
engine speeds always favors a dramatically reduced injection flow
rate, that is true on any diesel engine.  On the cylinder port two
stroke diesel the lower cylinder filling at low engine speeds tends to
favor slightly earlier injection start timing.  The lower cylinder
filling at low engine speeds also means that maximum torque is lower
at low engine speeds, meaning that less fuel is required.  The lower
cylinder filling at low engine speed also means less cranking
compression pressure so a higher compression ratio would be required
to get the engine to reliably start in cold conditions.  It might be
said that the reduction in cylinder filling at low engine speeds could
allow a diesel engine to run better over a wider range of speeds and
loads, but the requirement of a higher compression ratio is not such a
good thing.  Keeping the cylinder ports rather low would be desirable
for keeping the compression ratio reasonably low.  Bellow say about
21:1.
 
A simple regulator valve type injection pump with no advance mechanism
does have severe limitations, mainly that the engine tends to not be
able to run at high engine speed under a light load and the engine
tends to not be able to run well at low engine speeds under a full
load.  The reduced low engine speed cylinder filling of a cylinder
port two stroke diesel is a good match to the second part of this
simple regulator valve type injection pump limitation.  The injection
pump is not good for allowing full torque at low engine speeds, and
the engine itself cannot deliver full torque at low engine speeds.  A
slightly short stroke length also goes along with these limitations.
A short three or even three and a half inch stroke length works fairly
well down to rather low 2,000RPM engine speeds, but only under light
loads.  Under heavy loads such a short stroke length engine needs to
rev up a bit more to really run well.
 
It is sort of like the flexi 40mm Husqvarna conventional forks, single
piston Brembo caliper, drum rear brake and abrupt cylinder port two
stroke power delivery all going together to make for a bike of limited
performance but big fun.  A simple regulator valve injected cylinder
port single cylinder two stroke diesel dirt bike is similarly well
matched.  It does not make huge torque down low, but it can be made to
work fairly well both for delivering low fuel consumption at casual
cruising speeds and for belting out some big power up in the power
band.
 
The biggest problem remains an abrupt hit of the power band.  A
cylinder port two stroke diesel dirt bike would tend to have the
abrupt hit around 3,000 to 3,500RPM with overrev out to around
4,500RPM, although this could be tuned somewhat differently.  The
abrupt hit could just as easily come lower at 2,500RPM with maximum
power at 3,000 and over rev out to 4,000RPM.  For all out racing the
abrupt hit might come all the way up at 4,000RPM with a narrow, potent
and smoky power band from 4,000 to 5,000RPM.
 
There is also one construction detail that is important to consider
with a cylinder port diesel, and that is the requirement of a long
piston with a lower ring at the bottom of the skirt.  Without that
lower ring the lube oil would escape out the exhaust port when the
piston is at top dead center.  If the exhaust valves are instead
located in the cylinder head with only the intake ports in the
cylinder then this is not quite as much of problem.  Some loss of lube
oil into the intake ports would be acceptable as it just gets burned
during the combustion event.  Constant intake boost from a
turbocharger or roots blower also acts to keep the lube oil from
splashing into the intake ports when the piston is at top dead center.
The bottom line is that if the exhaust port is in the cylinder wall
then a long piston and bottom oil control ring is required.  That adds
complexity and weight, but it is not necessarily a deal breaker.
 
A great question to think about is what a cylinder port two stroke
diesel dirt bike would look like.  Would it have an expansion chamber
type pipe?  Yes, absolutely.  But it would be a different size and
shape because of the lower engine speeds involved.  More like the
giant multi bend organ on a 1987 Husqvarna 430 WR than the thin
megaphone look of a 2017 Husqvarna TX300.  The ’87 430 WR does tend to
start to build some substantial torque way down in the 3,500 to
4,000RPM range of engine speeds on slower flame front travel speed
gasoline.  That is very for a two stroke dirt bike, and getting closer
to what a cylinder port two stroke diesel would do.
 
Another thing to think about is the fact that too much exhaust
scavenging at low engine speeds tends waste fuel and cause dirty
exhaust emissions on a carbureted two stroke, and this has greatly
influenced pipe design.  Even a GDI case reed induction two stroke has
the problem of blowing more two stroke oil out the exhaust with too
much exhaust scavenging.  A two stroke with a pressure lubricating
system and a starting blower on the other hand does not run dirty or
waste fuel if it runs with too much exhaust scavenging at any engine
speed.  With a pressure lubricating system more exhaust scavenging is
always better, if it can be obtained without giving anything else up.
 
The rest of a two stroke diesel dirt bike engine might look rather
similar to a carbureted two stroke dirt bike.  The big difference
being the lack of a carburetor and spark plug.  Instead some type of
injection pump drive would be required.  The simplest injection system
would place the pump down on the crankcase roughly in the location of
the top part of the timing cover.  This would result in a high
pressure line running up to the top of the injector in the center of
the cylinder head where the spark plug goes.  The drawback of locating
the injection pump down low is that the high pressure line gets rather
long.  A longer high pressure line means more lag in the actual time
of injection and the injection start timing becomes too late at high
engine speeds.  Keeping the high pressure line shorter yields big
benefits on a simple injection system that does not have much of any
advance mechanism.  Especially for a longer three and a half inch
stroke length engine it might be important to move the injection pump
up higher to reduce the length of the high pressure line.  Taken to an
extreme the injection system would be a regulator valve controlled
unit injector without a separate high pressure line.  The drawback of
a unit injector on a two stroke diesel is just that it has to be
driven somehow.  A chain or gear set to transfer power up to the top
of the cylinder head seemingly unnecessarily adds bulk and weight up
high on the engine.
 
Probably the best compromise for a simple mechanically injected two
stroke diesel dirt bike would be a long lifter to drive the injection
pump off of the crankshaft.  This way the injection pump could be
located all the way up on the side of the cylinder head and only a
very short two or three inch long injection line would be required.
 
Really the biggest difference between a simple mechanically injected
cylinder port two stroke diesel and a carbureted case reed induction
two stroke would be that the diesel would use a lot less fuel.  Not
only is a diesel engine inherently more efficient than a gasoline
engine, but the fuel itself tends to be cheaper and have a
considerably higher energy density.  Two gallons of diesel oil might
weigh about 17% more than two gallons of gasoline, but when burned in
a diesel engine it would also tend to be able to push a bike several
times farther.  Especially in slow casual cruising the diesel powered
bike would just go a whole heck of a lot farther on the same volume of
fuel.
 
The difficulty in getting big power out of a small diesel engine is
also a big difference, but it might actually end up being less
significant than one might at first expect.  A good running diesel
engine has much lower peak cylinder pressures than a long stroke
gasoline engine, meaning that the diesel engine can actually be built
lighter.  A giant 700cc two stroke diesel might not be all that heavy.
The other thing is that the power of a diesel engine tends to have
better modulation and better power delivery than a gasoline engine.
The added control not only makes a bike faster in challenging
conditions, but better power delivery also tends to mean instant
torque is available when it is desired.
 
Taken to an extreme a cylinder port diesel two stroke with a narrow
power band and a high injection flow rate could get to be a bit of a
handful, but at least it would tend to be rather consistent.  The
biggest potential power delivery problem with an all out racing tuned
mechanically injected cylinder port two stroke diesel would be that it
would not run under light loads at high engine speeds.  This is
something that can be mitigated with the exact level of tune of the
injection system, but it is a valid concern.  As with some
mechanically controlled gasoline engines it might be important to warm
the engine up before revving it.
 
With the gas tank above the injection pump no lift pump is required,
so that keeps the fuel system simple, compact and light.  The
transmission clutch and final drive could be much the same as on a
gasoline powered dirt bike.  One big difference though would be that
the chain could actually be smaller on a diesel powered dirt bike.
Gasoline engines run harshly at low engine speeds, putting large shock
loads on driveline components.  A good running diesel engine is
smoother and is able to transfer more torque through the same
components.  If it is 700cc of diesel power though downsizing to a 420
chain might not be a good idea, stick with the 520 X-ring chain.
Clutch sizing would depend on just how the diesel engine ran.  In
terms of the static coefficient of friction the diesel engine can get
away with less clutch, again because it runs more smoothly with less
shock loading.  The real determinant of clutch sizing is it’s thermal
capabilities when being slipped aggressively.  An all out racing tuned
cylinder port two stroke diesel would tend to have an abrupt hit to
the power and a rather narrow power band.  That could require clutch
slipping.  The general tendency though would be for a diesel engine to
run better over a wide range of engine speeds with better modulation
of power delivery than a gasoline engine.  Better power delivery could
easily mean that the diesel powered dirt bike would not require much
if any clutch slipping.  If the clutch does not need to be slipped
aggressively then it can be smaller and still work great.
 
It is a similar story with the transmission.  A larger 700cc
displacement would seem to require bigger gears and bigger bearings,
if however the diesel engine ran well it might get by with a
surprisingly svelt transmission.  Another thing about the transmission
is that banging gears at lower engine speeds does not result in nearly
as high shock loading as banging gears at higher engine speeds.  The
slower spinning but larger displacement diesel engine probably could
get away with a rather light and compact five or six speed
transmission.  It just requires keeping the rotating mass of the
engine down, generally a good thing for smooth clutch engagement
anyway.  Diesel engines tend to be very resistant to stalling, but
that can be less true of regulator valve type injection pumps than
metering collar type injection pumps.  It just depends on exactly how
the injection system is setup.
 
Kick starting a 700cc diesel does not seem to be an appealing
prospect.  Electric start sounds like a great idea.  A battery and
alternator are required anyway to power the starting blower.  Adding
the electric starter and doing away with the kick starter does not
seem to add much weight at all, KTM has made that quite clear in
recent years.
In the end though a case reed induction two stroke has the potential
to be considerably lighter.  They might be difficult to get to run
well, but it has the potential to be a very compact engine type.  As
long as cars are burning gasoline it would stand to reason that
carbureted or GDI case reed induction two strokes of some type will
continue to be popular.  If for no other reason than the fact that so
many people think they have the potential to be light weight and
powerful.
 
Perhaps AMA could introduce a self contained class in Enduro racing.
Instead of fuel stops the self contained class would have to finish
the entire course on one tank of fuel.  That would certainly encourage
diesel engine development.  A good running diesel could get through a
150 mile course pretty easy on two gallons, where most gasoline
powered bikes would need at least four gallons.  A 250F tuned to run
well down in the 4,000 to 6,000RPM range should be able to do better
on gasoline also, but as it stands today the 250F bikes have giant
camshafts and won’t make torque bellow about 7,000RPM. That uses a lot
of extra gasoline and dramatically reduces range at low to medium bike
speeds.
 
Even with efficiency oriented tuning a gasoline engine is never going
to be able to go as far though simply because the gasoline does not
have as much energy content per gallon.  Regardless of what is done
with 250F class bikes diesel power would always have somewhat of an
advantage when the required range gets up towards the 200 mile
distance.
 
Ultimately what it comes down to is that dirt bikes can be either two
strokes or four strokes and dirt bikes can be either gasoline powered
or diesel powered.  There are some reasons why two stroke and diesel
go together well on a dirt bike, but a low pressure GDI two stroke
with a pressure lubricating system and a starting blower is a great
idea also.  The simplest dirt bike engine is actually a carbureted
four stroke.  A mechanically injected four stroke diesel can be very
simple also, but a higher revving carbureted four stroke obviously has
a lot more performance potential.  About the only type of engine that
is really dramatically worse is a carbureted two stroke, simply
because it is so dirty.
 
The most important thing to keep in mind is that gasoline engines run
on gasoline and diesel engines run on diesel oil, and there is only
very limited room for any cross over between the two.  A high pressure
GDI engine might run with time of combustion injection to increase
torque down at low engine speeds, but it is sort of ridiculous to try
to burn gasoline under conditions where diesel fuel could be used.
Similarly there are ways that dual fuel systems can be used to burn
diesel oil in high revving high output gasoline engines, but switching
back and forth requires fairly sophisticated controls of some kind.
 
A turbocharged dual fuel engine with a high pressure direct injection
system could potentially do some interesting things.  Under heavy
loads at low engine speeds with lots of boost pressure diesel fuel
could be injected at the time of combustion and the engine would truly
be running as a diesel engine.  Once the load was reduced though the
engine would have to switch back to gasoline and use the spark plugs.
In fact the engine would probably need to switch back to gasoline
BEFORE the engine load was reduced to clean up the spark plugs and get
them ready to fire.  If the spark plugs got oily running on diesel
fuel then the engine would just stall once the load was reduced to
where the fuel would not ignite when injected.
 
It would actually be easier to keep the spark plugs clean by running
diesel oil at higher engine speeds as a gasoline engine.  A dual fuel
gasoline engine does not require direct injection to run diesel fuel,
what it requires is switching back to gasoline to support reduced
loads or low engine speeds.  Some type of EFI system, either port
injection or low pressure direct injection, is highly desirable for a
dual fuel engine.  Being able to instantly switch back to gasoline is
very important for the usability of a dual fuel engine.  A carbureted
dual fuel engine requires anticipating the need to switch back to
gasoline, and that is much more difficult.  With a set of injectors
for gasoline and a set of injectors for diesel fuel the engine can
instantly switch back to gasoline as soon as the throttle begins to
close.
 
There are some reasons why a dual fuel engine might use port injection
for the gasoline and direct injection for the diesel fuel, but this
would also tend to be rather confusing.  The main reason to use port
injection for the gasoline is just to keep the second set of injectors
out of the combustion chamber.  The main reason to use direct
injection for the diesel fuel is to increase power output.  The
confusion comes from the fact that the engine would be a GDI engine
running as a gasoline engine, but it would be running on diesel fuel.
It is understandable that many people would have somewhat of a hard
time understanding that it is still a gasoline engine, a spark
ignition engine, when it is running on diesel oil that is being
injected directly into the combustion chamber.  It really is still a
gasoline engine because it has to switch back to gasoline whenever the
engine load is reduced to the point where full flame front travel mode
operation is required.  Each time the throttle is closed or the engine
speed is reduced bellow about 3,000RPM the engine has to switch back
to gasoline.  A dual fuel engine won’t start without gasoline, and it
can’t really be used without gasoline.  What a dual fuel engine can do
though is burn diesel oil for all heavy acceleration and sustained
higher speed cruising.  The diesel oil burns hotter and has a higher
energy density for better fuel mileage and higher power output, and
also would tend to cost considerably less than gasoline.  Diesel oil
is a better fuel, it is just harder to make use of.  Burning diesel
oil in a gasoline engine is a very clean way to burn diesel oil, it
just only works at higher engine speeds under heavy loads.
 
For a dirt bike a dual fuel engine does not seem appealing simply
because of all the extra complexity and all of the extra weight of two
parallel fuel systems.  It would be possible to cram all of that duel
fuel EFI gadgetry onto a dirt bike, but there are quite a few reasons
why it would not exactly be desirable.  Having dual fuel tanks makes
refueling more complicated and slower.  Accidentally pour diesel oil
into the gasoline tank and the bike won’t run.  Just 20 or 30% diesel
oil in the gasoline normally would not prevent the engine from idling,
but it would not accomplish anything either.    It would just be
smokier and dirtier idling and under low power output.  More than
about 50% diesel oil mixed with the gasoline pretty much guarantees
that the engine will not idle.  And even 10% diesel oil mixed with the
gasoline can cause difficulties with starting.  Mixing the diesel oil
and gasoline is not desirable in any way, except of course for a
carbureted two stroke which could potentially use 3 to 5% diesel oil
instead of two stroke oil.  The only way a dual fuel system is really
useful is when separate tanks are used, and this raises the question
of how much of each to carry.  If you run out of gasoline the bike is
totally stuck, so this would tend to bias the fuel storage to more
gasoline and less diesel oil.  If the bike carries only a small amount
of diesel oil then the purpose of the duel fuel system is somewhat
defeated.  It can’t burn much diesel oil because it can’t carry much
diesel oil.
 
If a dual fuel system was for some reason desired on a dirt bike it
certainly would be possible though.  The two tanks would probably be
about the same size.  On most normal rides several times more diesel
oil would be burned than gasoline so that the gasoline tank would
still be substantially full at the end of the ride.  That extra
gasoline would however be available to get back the last few miles on
a long ride even after the diesel oil ran out.  In very challenging
slow going conditions hardly any diesel oil would be burned, so this
is where the large gasoline tank would be important.
 
This would also mean that there would be a large incentive for the
engine displacement to be fairly well matched to the types of intended
ridding.  A huge 500cc engine on single track trails would have a hard
time burning much diesel oil.  A smaller 150 to 300cc four stroke
engine would probably be much more appropriate for most types of
single track riding.  The smaller engine would burn less fuel overall,
and also it would burn proportionally more diesel oil in tight
challenging conditions or in slower casual cruising on larger trails.
 
-Michael Traum
 

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