FUEL USAGE WHEN DOUBLING HORSEPOWER

[FLATTOP]

We ran a Porsche normally aspirated 2.7 lt engine. The power output of this eninge was 250 hp. We have recently installed a new 3.6 lt engine (supercharged) that generated 500 hp. I need to know whether I can assume that when the horsepower is doubled will the fuel consumption increase at the same rate. For exampled if we used to run with .8lt per lap will we now run at 1.6 lt per lap?

Thanks

Johan
ALMOST BROKE RACING

http://www.almost.co.za

 

[SomptingGuy]

Can you stay on the gas for the entire lap?

 

[crysta1c1ear]

Is this a question about the supercharged engines, the racing circuits, or the two combined?

 

[GregLocock]

You've got two effects. First the supercharger will worsen the efficiency of the engine. WAG: NA 2.7=250, NA 3.6=310 , SC 3.6=500, so you are generating 190 hp extra at the flywheel. To get that your s/c drive is probably somewhere between 40 and 120 hp extra, so your crankshaft power is of the order of 580 hp, so at full power you'll be burning more than twice as much (580/250).

But, you won't be at full power for as long, so you'll get some benefit.

On Donnington I get a doubling of power is worth about 6% off the lap time, according to LapSim. You'll also spend more time braking, and pretty much the same time going round corners, so more than that time is subtracted from full power, call it 10%.

So I'd guess you'll use 0.8*580/250 *.9=1.7 l/lap

So altogether I'd expect the new engine to need

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[NormPeterson]

Don't you use a little more fuel above and beyond that due to the typically higher BSFC with forced induction?


Norm

 

[patprimmer]

Sometimes you run rich to cool the charge and avoid detonation.

Also, even at part throttle, there is some parasitic loss to the supercharger, so more crank power is needed to produce the same wheel power out of a corner.

Also a much more powerful car might require a different driving style to utilise the extra power. For instance at lower power you might take a line that washes off least speed in the corner, but with more power, you might take a line which scrubs of more going in, but lets you put down more mid corner and coming out. Watch F1 vs Formula 1 lines and techniques, or circuit race vs sprint car.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[FLATTOP]

Hi guys. Thanks for the wonderful and prompt responses. I shall respond in the order you posted.

1) No we cannot run full throttle all the way. The reason for this question was that our fuel tank is of limited capacity and we are trying to establish what size tank we would require bearing in mind the car would now be more thirsty.

2) GregLocock - Your estimate of a 6% decrease in lap time fall very close to our expectations. I valued your insightful comment on more braking will be required, with the about the same corner speed.

3) Patprimmer - I was also expecting a different "feel" and driving style. This is my first post to this forum and I was surprised to get any response never mind the subtle touch you have of understanding how cornering would be affected with a more powerful motor (once the suspension has been sorted.)

So I guess it is now down to building a new tank. For what it is worth you can see our fuel cell enclosed in an aluminium box on our website. Address below.

Thanks


Johan

http://www.almost.co.za

 

[WGJ]

What sort of fuel delivery system are you going to use?
Are you running returnless, full-return, PWM fuel pump?


Bill

 

[FLATTOP]

Bill - Fuel system follows the conventional Porsche 930 Turbo system. That is: It has a fuel pump at the tank and then a second pump (high pressure) at the rear close to the engine. Any excess fuel is returned to the tank.

Regards

Johan

 

[WGJ]

Hello again FLATTOP.

The reason I asked about your supply system was to see whether your plans were likely to place a restriction on power output by your fuel supply capability.

I will apologise in advance if I am trying to teach you to suck eggs.

Assuming the 580bhp that GregLocock suggested (engine + supercharger requirement), I estimated that for full engine power alone, you would need a flow of 185 litre/hour.

Since you're using a full return system, an amount of flow to ensure that the pressure regulator is working in its linear range is necessary. If it's a Bosch Mini-reg then about 20 litre/hour is OK. If it's one of the older, large, Bosch regs then 40 litre/hour is nearer the mark.

Some recirculation flow is needed to purge any air you pick up in the ank, say 20 litre/hour.
Finally (almost) some allowance for vapour generated nearer the engine ought to be added, say another 20 litre/hour.

Since this is a racing car, no allowance for fuel pump wear out is being made, as it would be if it were a road vehicle accumulating many miles.

The final fuel flow figure is now 265 litre/hour at whatever your fuel rail pressure is.
I'll assume that as 3 bar (EFI typical)+ 1bar boost = 4 bar.

May I suggest you look at your fuel pump wiring?
Systems I have worked on with this sort of performance had twin (parallel) electric pumps to make sure the pressure/flow limits were exceded.

Typically, I used either Pierburg or Bosch pumps and the total current required for full engine power was often over 18 amps, and in one case 25 amps.

Check your wiring. If, at full throttle, your getting less than 12 volts at the pump terminals, you're restricting the engine fuel flow. Thicker and shorter wires will be necessary.
Are you running a total loss electrical system (either no alternator or a very small battery)? Does it have the capability to supply a constant 18A+ for the fuel system alone?

I recently saw a VW Beetle dragster with a turbochrged 2+litre engine and a 5 bar fuel system. The chap already had two Bosch 5 bar fuel pumps to supply fuel, however, their power went via a realy and a couple of feeble wires, so he wasn't getting anywhere near the flow he needed and was wondering why.


Bill

 

[FLATTOP]

Bill

Thanks for your detailed and thoughtful answer. I shall investigate all aspects as indicated by you.

Regards