andrewaust wrote:I've always been curious if a turbo would work on a single diesel as many have no form of throttle valve
There is no throttle valve because it does nothing useful in a diesel. In fact it only does harm by increasing the intake pressure loss. Mercedes-Benz used a throttle plate in some W124 diesels to increase EGR "effectiveness" at idle, but as EGR is also extremely harmful to the engine, the throttle plate did A LOT more harm than usual.
A throttle plate also does no good for any turbo - turbo gassers need a blow-off valve setup in some form to prevent compressor stall (
EXTREMELY harmful to the turbo, can rapidly snap the turbo shaft) from occuring when the throttle is closed. Also again it only causes heavy losses in any case.
andrewaust wrote:Diesel engines have a torque curve around 2200 rpm for smaller engines, truck engines etc can be much lower! So what has this got to do with a turbo on a single? To get good power, torque and efficiency from a diesel, boost should start at its torque curve or it will lag, being a real slug.
You are
seriously misunderstanding how a diesel engine produces torque (and thus power).
An NA diesel has a nearly flat torque/RPM characteristic - there is still a torque "peak", but the "peak" torque is only 5-15% greater than low- and high-end torque (various losses at low and high RPMs decrease the available output torque slightly). An idealized (nonexistent) NA diesel has a perfectly flat torque characteristic all the way from idle to the redline due to the absence of these losses.
A diesel engine is a heat engine - it converts heat energy to mechanical energy (used to drive the bike). The cylinder can only hold so much air, and you can only burn so much fuel in that air before it will refuse to burn completely and belch tons of black soot like... well, like a smoky diesel.
In any case the output torque is approximately proportional to the amount of fuel burned per engine cycle. And power = torque * RPM.
Because the amount of air available per cycle is nearly independent of RPMs, the max amount of fuel that can be burned is also nearly constant, and thus the available torque is also nearly independent of RPMs.
We can't really increase the max RPMs much - the heavy internal parts of a diesel engine don't take well to overspeeding, and there are other issues pertaining to the injection system which make this even more infeasible.
So how do we increase the available power? By increasing the available torque. And to do that you need to burn more fuel per cycle.
But to burn more fuel you also need more air. Since the cylinder displacement can't really be changed, you need to increase the density of the intake air. The only practical way to do that is by pressurizing it.
However, compressing air in such quantities takes a tremendous amount of power. That power has to come from
somewhere.
Enter the turbo.
A turbo is also a heat engine in its own right. It utilizes the
waste heat contained in the exhaust gasses as a source of great power, and uses that power to smash a lot more air into the cylinder than would ordinarily fit. This allows much more fuel to be burned, and more power to be generated.
However, there is a catch. The turbo utilizes a centrifugal compressor and a centrifugal power turbine. Centrifugal turbomachinery is highly sensitive to the shaft RPMs - in fact, the actual performance varies with the
third power of the shaft speed. So it needs to spin fast. Extremely fast. To the tune of 3000 revolutions
per second. At such speeds the turbo itself is working at a power level of roughly a third as much HP as the diesel engine itself. That's tens of HP through a 5...7mm shaft.
To reach such RPMs the turbo needs a VERY high exhaust gas flow rate. Which is why it's extremely critical that
the turbo size is properly matched to engine displacement. A turbo that is too small will start to develop max boost very early on, but at high engine RPMs, the airflow through the turbo inlet will approach the speed of sound, and no more air flow is physically possible - the engine is now being severely starved for air, and so the top end performance suffers enormously.
On the other hand,
a turbo that is too big will never reach its optimal operating point, and so the performance will also suffer greatly - the max boost (and thus power gain) will be mediocre, also only being reached close to the redline, and there will be massive turbo lag.
andrewaust wrote:I set up the turbo today very crudely, nothing seemed to happen till I reached the high rpm limit of the engine, so results were not that impressive. At idle the turbine is hardly spinning that you
can actually see the vanes spinning around.
I'm not that really concerned if I cannot get it to work as this little turbo will be used on another multi cylinder project, before I commit to going further I'd like to see how others have gone.
At this point there is nothing for me to add. It is painfully clear that your turbo is hopelessly mismatched to the engine it's attached to.
andrewaust wrote:Maybe a small blower is more effective? That I'll leave for someone else to tinker with
.
You've hit the nail right on the head. The problem is, there is not really a "smaller blower". The
smallest turbo commercially available - can't remember the designation offhand (I think it was a Garrett turbo though), but it has appeared at least once on this forum - is still
so big that the 0.8L V-twin punsun is BARELY big enough to work acceptably well with that turbo.
I hate to burst your bubble, but there is just not a turbo available that would be an optimal match with these single cylinder diesels.
The highly pulsed nature of both the intake and exhaust flow in single cylinder engines also hurts performance a lot. Using exhaust and intake plenums would alleviate the issue somewhat, but then you are introducing excessive turbo lag into the system.
And finally, something that is really obvious yet many people fail to realize it -
if you turbo a diesel you MUST increase the max fueling to actually gain any power. Really it's just a reiteration of what I've said above.
On passenger cars a special pneumatic actuator is used (generically referred to as an ALDA) on the injection pump which automatically increases the max fueling proportionally to boost pressure. On these single/twin cylinder engines you would need to homebrew such a setup yourself.
