Small diesel engine turning supercharger

[APU]

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 ?

[drmiller100]

Originally Posted by APU

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.

[APU]

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

[ConnClark]

Originally Posted by mechman600

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.

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.

[drmiller100]

Originally Posted by ConnClark

As long as you don't expend more energy creating the boost than it returns increasing boost will always improve efficiency in a diesel. I.

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

[drmiller100]

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.

[mechman600]

Originally Posted by ConnClark

As long as you don't expend more energy creating the boost than it returns increasing boost will always improve efficiency in a diesel.

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

[hawk2100n]

Originally Posted by drmiller100

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

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

[ConnClark]

Originally Posted by drmiller100

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

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

[mechman600]

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.