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.

Supercharger guys tell me that inefficiency with roots blowers are due with the throttle demands. Roots blowers are best at fixed RPMs. If all the power requirements are meet without changing RPM then the best efficiency should be achieved. The same load can be pulled with a vehicle with a much smaller displacement, V6 instead of a V10 or V8. when the extra power is no longer nessesary the diesel engine simply won't be started.

Has anyone looked into the shaft RPM of the air conditioning compressor and alternator that works most efficiently ? I can make the pulleys for the system that will put all the components in their "sweet spot".

supercharging an engine does not make it more efficient.

but if you believe in your dream, then build it!!! Good luck!

twin turbo's all the way !
The reason they use twin turbo's is that one of the turbos is smaller so it has very little turbo lag and one is bigger and pumps larger volumes of air.

Not if its a diesel :)

Usually true. But adding boost to a diesel will not necessarily increase efficiency either if it isn't too rich without boost in the first place. It will not necessarily increase power either, unless you fuel it more. I've seen it a million times on the dyno - putting vice-grips on the wastegate line to increase "power" (as if boost is an indication of power) only to see power actually drop slightly.

How about an electric super charger? There would be virtually no lag, and it would only spin when the extra air was needed.

An electric supercharger could control throttling as well. A positive displacement supercharger (ie roots style) not turning would provide plenty of restriction, eliminating the need for a throttle plate. It could be spun up enough at any given time/situation to control manifold vacuum/pressure. Controlling it all would be a challenge. Probably best left to the OEM engineers.

run the numbers on how much power it takes to compress the needed air to the boost you want.

Then convert the horsepower to watts. Then divide by 12 volts, and you will see you need thousands of amps to make it work.

Good luck!

3800 cc engine, 14.27 psi, 6000 rpm, Eaton m90 supercharger 19.5 hp
convert horsepower to watts 19.5 horsepower = 14541 watts
14541watts/12 volts = 1212 amps

This is the reason I'm looking for an internal combustion solution .

Even 2 to 5 lbs boost would add pep, I bet you could make a home brew supercharger that went up to 2 to 5 lbs. My 1.9 turbo diesel uses 15 lbs steady to accelerate hard but in that case I am not running the car economically, but at 2 to 5 lbs I am.

During high torque demand, accelerating tow vehicle, uphill and passing, the auxiliary engine would run providing boost to the engine. My target is 6 psi, this should provide approximately 33% more torque. The auxiliary engine would be shut down during highway cruise and only restarted when required. The majority of the time the main engine would be run naturally aspirated. Would running the auxiliary engine during cruise, with taller overall gearing lowering rpm, increase or decrease the main engines fuel consumption ?

pumping air for the fun of it will decrease mpg.

what size main engine and what RPM are you thinking, and I can run the numbers for you as to how much horsepower it will take.

Actual Weight Tow Vehicle (GVW) 5,930
Theoretical Tow Rating (GCVWR - Dry Weight) 8,150
Total 14,080

Dodge 3.9 liter V6
SAE Net Horsepower @ RPM 175 @ 4400
SAE Net Torque @ RPM 225 @ 3200
4 speed auto transmission
3.91 rear end ratio

Dodge rates this vehicle with a V8.

Dodge 5.2 liter V8
SAE Net Horsepower @ RPM 225 @ 4400
SAE Net Torque @ RPM 295 @ 3200

As you can see the horsepower is marginal and the torque is insufficient. If we want to travel in any area that is at all hilly the vehicle is unable to climb at all, but the fuel efficiency is good.

My goal is to keep the fuel economy while increasing it's hill climbing ability.

Thanks

As long as you don't expend more energy creating the boost than it returns increasing boost will always improve efficiency in a diesel. If you had a way to generate boost that was 100% efficient it would improve efficiency and power up until the point the air starts to liquify under the pressure. The main reason more boost improves efficiency is you have more working fluid mass to heat and thus more fuel energy going into heating the working fluid (air) and less energy going into heating the engine block.

You will however get diminishing returns as you increase the boost.

if only the laws of thermodynamics didn't interfere, we'd all be there.

from

Air flow (CFM)


the v6 uses 242 cfm.
the v8 uses 323 cfm.


lets say our goal is to take the v6 and make 250 horsepower at the same rpm.

this is about half again more horsepower, which means we need half again more CFM (before compressor).

so, we need to compress 360 cfm to 1.5 BAR.

that is only 9.3 horsepower.


Horsepower required to Compress Air


Oops. we need to probably assume the compressor is only 50 percent efficient, so a better answer is 20 horsepower.

for those using electric motors, 20 horsepower is (20 * 746) = 15000 watts which is only 1100 amps for a 12 volt system.

anyway, a better answer is to buy a modern chevy 5.3 motor. They get GREAT mileage as they run just fine at 1400 rpm for mileage, but rev to high rpm for horsepower.

320 horsepower at 5200 rpm. Cool motors.

Um....circular reasoning?
An engine is always more efficient when it is uses less fuel.:D

Drrmiller is right. Thermodynamics will prevent this idea from seeing any real mpg gains. (the first law of thermodynamics says that energy cannot be created or destroyed, only changed) The small engine would have to have significantly higher BFSC values which won't happen. Even if it is diesel, small industrial engines are relative pigs when it comes to fuel consumption. There is no free energy. A turbo charger has the smallest impact but the gains from it aren't realized unless you can move to a smaller, more efficient engine with significantly less friction. Think turbo v6 or i4 a-la powerboost ford f-150. A turbo v8 that is otherwise stick will use more fuel most of the time. The only free energy you can actually get is solar. If you're in a pickup, then you can install a tonneau and cover it with solar panels and mount some LiFe batteries in the bed for a lightweight setup and delete the alternator at the same time. Then a sufficient motor could power a blower but it would be complex.

And there will always be losses associated with creating boost. You won't make 50 hp from 5 hp. It just doesn't work like that at all. You can provide enough air for your engine to make the 50 hp but the engine will have to add a lot of fuel for that 50hp

Fortunately on a diesel the laws of Thermodynamics don't limit us too much. Adding 10 psi of boost with a roots blower can still improve fuel economy.
http://aerade.cranfield.ac.uk/ara/1936/naca-tn-569.pdf

Edit more proof (page 18 in the pdf)
http://naca.central.cranfield.ac.uk/...report-577.pdf

Proof? Those two documents are 74 and 76 years old and deal with pre-combustion chamber diesels and natural aspiration. Of course adding 10 psi of boost pressure would have made a monumental difference back then!

Today we have direct injected diesels that are already boosted. In a VW 1.9 TDI engine, raising boost levels from what they already are will not increase economy. For example, the reason for the wastegate is to a) allow a smaller turbine for quicker response and better low-end performance, and b)(the relevant one here) to prevent this smaller turbine from over boosting in the top-end and reducing economy.

wrong.

is the goal more power, or more "effiicent"????

those are precombustion chamber engines, and run with incredibly inefficient parameters.

The OM 617 in your 300SD is one of the last common engines with precombustion chambers. I had a 300TD. But the best way to drive these cars for max fuel economy is below the onset of boost when possible, which is hard. Same goes for all diesel engines. Turbos are for power. I usually got around 27 mpg in my wagon. It's still in the family. My dad is hanging on to it but he and I bought it together when I was in high school. 230,000 miles on it now.

Exactly. That was the best technology they had, when super fine spray patterns and high injection pressures were not yet possible because of lack of machining tolerances and "pre-cups" were the only way to make it work. Injection pressure was probably peaking at 4000 psi at rated HP, where even an older TDI (rotary pump) will hit 15000.

This is because when boost isn't there, the air/fuel control in the fuel pump limits rack travel, AKA fuel quantity per stroke. Electronic engines do a variation of the same by limiting mm3 per stroke until boost reaches a certain level (smoke limiting) and EGR flow reaches a certain level (NOx limiting).

More boost raises engine efficiency. Had to find it in the internet archive but here it is and it says the same thing as the earlier papers (From 2004 on a Direct Injection engine).
http://web.archive.org/web/200611080...4/A6_1_022.pdf

As far as efficiently driving a turbo diesel engine you want to drive generating low boost pressures. That shows the pistons are extracting as much power as they can before they dump the rest of the energy in the combustion gases to the turbine.

As far as the engine itself you want it to generate the most boost as efficiently as possible. The purpose of the waste gate is to keep boost pressures in a range to avoid wear and tear on the engine and keep the boost in an efficient range of the compressor map and keep back pressure in check.

So, if you are trying to increase the boost setting on your current system you probably won't improve efficiency. If you improve it by using a more efficient turbo you can raise boost and get better fuel economy