Reducing PV work in a diesel engine using a throttle body.

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Peter
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Reducing PV work in a diesel engine using a throttle body.

Post by Peter »

So I've been working on this 86 VW, because it gets around 40mpg. Which is far better than anything else I've ever owned, well maybe except motorcycles.

The theory is/was that the engine was doing a lot of excess work compressing air that it doesn't need except at full throttle. I was considering trying to control a drive by wire throttle body, so that it would be close(r) to running a stoichiometric mixture. I've decided that it would never pay for itself, but I don't have total faith in my math or assumptions. So I'm here for peer review.

Work equals the integral of pressure with respect to volume. Where Vf is the volume at TDC, and Vi is the volume at BDC.
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Since the pressure is a function of the volume:
CR is the compression ratio, gamma is the specific heat of the working fluid(1.4 for air), Vc is the combution chamber volume with the piston at TDC, Ptdc is the pressure at TDC, and PBDC is atmospheric pressure assuming the volumetric efficiency is 100%, or you're at sea level and VE is about 75%.
Image
I'm working with a 1.6L 4 cylinder engine with a compression ratio of 23:1 at 7200ft in elevation. The results are negative, because the work is going into the system.
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At 75mph I assume the engine is doing about 2300RPM(guess) which is 4600 compressions per minute. Which is 276000 compressions per hour. With a savings of 0.87 J/Comp you get 240kJ/hr, and for 1.88 J/Comp you get 519 kJ/hr of savings. I used a heating value of 43200 kJ/kg, and a density of 0.8 kg/L for diesel. So I get 130909 kJ/gal for diesel. Then I assumed the engine is 20% efficient(too high?). Which gives me a mechanical energy of 26181 kJ/gal. 240 kJ/hr / 26181 kJ/gal = 0.0092 gal/hr of savings at 2/3 throttle, and 519/26181=0.0198 gal/hr of savings at quarter throttle. The longest trip I've ever taken was 31 hours one way, so 62 hours round trip. 62*0.0092= 0.57gal, and 62*0.0198= 1.23gal. So about 0.5 to 1.2 gallons of saving for about 3700 miles of driving depending on how much throttle it takes to go 75mph.

Fred told me that he thinks that the pistons making vacuum against the throttle body would also lower the efficiency. I think that would be negligible in comparison to my already negligible fuel savings.
:-p
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Fred
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Re: Reducing PV work in a diesel engine using a throttle bod

Post by Fred »

The thing with work done compressing the mixture, is that it's done again in reverse with the mixture pushing the piston down out of the way on the power cycle (even if no combustion is involved). However on the intake stroke you've got up to 100kPa of differential pressure pulling in the wrong direction on the piston. On the exhaust stroke, however, you've got no solid force, you open, spent gas rushes in to fill the vacuum left by the intake cycle (again, assuming no combustion) and you end up just wiggling ex gas around, it doesn't such the piston up because there is no seal there.

I'm probably way off base here, so keen to hear the input of everyone else with an opinion :-)

Fred.
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johntramp
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Re: Reducing PV work in a diesel engine using a throttle bod

Post by johntramp »

Fred wrote:The thing with work done compressing the mixture, is that it's done again in reverse with the mixture pushing the piston down out of the way on the power cycle
Would that only be the case if the excess compressed air was allowed to expand back to atmospheric pressure before the exchaust port was opened? As once the port is opened, any energy stored in the compressed gas would be lost.
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Re: Reducing PV work in a diesel engine using a throttle bod

Post by Fred »

Yes, which it is, because you only open the exhaust port around BDC so that you can push it out.
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