Small diesel engine turning supercharger
 

Has anyone experimented with using a cheap diesel 13hp engine to power a Eaton supercharger to increase the VE of a larger main engine. Towing applications maybe ?

Thinking the diesel could be run a constant RPM. Supercharger could be run at the best part of its map. Maybe other other power robbers could be powered by the diesel too IE: alternator,air conditioning.


What do you think ?

D Morgan
WEGMOR

why not just use the main engine?

i guess it could be done, but, i don't think it makes sense to do it.

mechanical driven superchargers are very inefficient.

turbos use the exhaust heat to do a lot of the work - much more efficient.

what doug said.

i really don't understand why anyone uses mechanical superchargers over turbos. there is the lack of lag, but, with modern turbos, that's not much of an issue anyway. engines run better with a certain amount of back pressure anyway, so, it's free power.

The roots supercharger would be run at a constant RPM, the speed at which its Volumetric efficiency would be maximum. The diesel could then run at its rpm where it's fuel efficiency would be greatest. This all being done while not producing any more horsepower requirements from the vehicle's drive engine. All of the additional power output of the engine could then go into propelling the vehicle. Taller gearing could then lead to lower RPM without sacrificing the superchargers efficiency at the cost the fuel consumption of the small diesel, which I believe to be quite stingy.

D Morgan
WEGMOR

How big a diesel are we talking about? For 13 hp, if I recall my small diesels right, you're looking at at least 500cc of diesel, which would add about a gallon per hour in terms of fuel consumption (diesel) at a constant 13 hp load... as opposed to a much simpler to set up turbocharger system which doesn't add any fuel cost besides the fuel needed to keep the charge cool.

That extra fuel cost could be offset by running the accessories straight off the small diesel, yes (I've often wondered if this was the future of electrics... having a small separate ICE to run the air-conditioning! :D ), but I don't see the benefit versus complication... would be a cool project though.

If you really want a positive displacement supercharger you're better off running it straight off the belt because a separate engine would be a lot of weight, and would probably lack in efficiency compared to the main engine anyways. Belt drive losses are very small, and the efficiency of a Roots blower depends a bit more on pressure ratio than speed.

If you want fancy ideas about using a Roots blower, some things to consider would be electric drive (so it can be operated in reverse for power generation off intake vacuum, but on a diesel this is not very useful), and a CVT connecting it to the crankshaft for direct boost control or the ability to recover throttling losses on a gasoline engine.

This would be possibly for a large tow vehicle, so the extra weight would not be a factor. If this vehicle is already consuming 10-12 gallons per hour then the additional consumption 1 gallon per hour of the auxiliary engine could increase the efficiency of main engine 3-4 gallons per hour. The power accessories running at constant rpm might increase their efficiency also. Has anyone looked into what speed the accessories run most effectively? I’m also interested, has anyone used a CVT to drive a blower ?


Thanks for the input.

D Morgan
WEGMOR

Your accessories would be perfectly fine running off a diesel generator running at 2000 or so rpm.

Adding boost to an engine will never increase the efficiency of the vehicle, even if it is from a 100% free source, which is never possible anyway. The only time boost helps is if you use a smaller than normal engine boosted to "normal" power levels.

Put it this way: an engine with a supercharger and an identical engine without a supercharger at the same load (say steady state cruising) will have the same manifold pressure (ie vacuum) at said load, but the pumping losses will be greater with the supercharged engine because it will take more throttle restriction to maintain the same manifold pressure.