Fuel Dynamics

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Does anyone know to what extent Polyphony Digital simulated fuel dynamics, such as consumption rates and added weight? I've done a few tests and came up with some things, but I'm still wondering about it a little.

Dynamic fuel consumption is simulated, to a degree. Both short shifting and going easy on the throttle will reduce the amount of fuel used. You probably already thought you knew that, but I've confirmed it with a few tests I did. As to which saves more fuel, short shifting or low throttle, it really depends on the situation and how you want to drive. I did a test on the Test Course in which I had a cruising speed of 330km/h; I tested maintaining that speed with throttle management with both normal gearing, and high gearing (meant to establish a cruising gear). The difference in fuel use was negligible.

I also tested the affect of oil on fuel usage. I took my 787B, fresh out of the LeMans 24 hours, out to Nürburgring with 5500km old oil. I used 13 liters for one lap. Then I changed the oil and ran another lap. I used 14 liters that time. Now, the drive was a bit different both times, so there could be some error to be had in that, but I would have expected fresh oil to make the mileage better than before. This means that either the effect of oil on fuel usage is negligible, or that fuel usage is directly related to how much power the car is putting out rather than its actual efficeincy. To test if the latter is true, one would have to put various upgrades on the car that don't imply more fuel being injected into the cylinders, such as muffler/air cleaners or intercoolers, and see how those affect it. I have not done that yet.

Also, as I was doing the LeMans 24 Hour race, I noticed aboslutely no difference between the performance of my car with a full tank versus an empty tank. I wasn't expecting things like goofy weight transfer from the liquid fuel sloshing around, but I would have at least hoped for a lightened car...It wasn't a rigorous experiment, I just didn't feel any difference. If someone wants to test it further, feel free.
 
In real life, fuel consumption is much more dependent on throttle input than shift points ...
 
Thanks for the testing! Sounds to me like fuel usage is proportional to the amount of power being used (or the total amount of work done=avg. power*time). This explains everything in your post: driving slower on the test course is more efficient due to less aerodynamic drag, which reduces the amount of work needed to lap the course (W=F*D), and gearing has no effect when speed is held constant because the power needed to maintain that speed is constant. Oil change increases fuel consumption at the 'ring because the car has more power, and the increase in power is proportionally greater than the reduction in lap time.

In real life, taller gearing/short shifting is almost always better for fuel consumption due to the reduction in throttling losses. Every time you let off the throttle, you are choking the air flow into the engine and the engine has to use some power to pull air past the throttle.

In the game, it seems the best way to save fuel is to let off the throttle near the end of straights, as you will not lose much time but will save a good amount of fuel. Also, when I was doing the new york enduro in a stock SL65 AMG I thought the car seemed lighter with low fuel, and my lap times reflected this, but I might have just been learning the course. have you tested this with B-Spec?

Finally, intercoolers and intakes (air filters) can in fact imply more fuel being injected into the cylinders (in real life). Intercoolers because they cool the intake air, allowing more into the cylinder at the same pressure, and therefore more fuel (assuming the injectors can keep up). Intakes because they function in the same way as a throttle, introducing a pressure drop in the intake tract (the upgraded one presumably has less pressure drop). With less pressure drop, the pressure in the cylinders is greater, again allowing more air and fuel. However, the engine model in GT4 doesn't seem to be anywhere near this involved, especially as larger turbos don't seem to shift the power band like a large turbo would in real life, and upgrades besides NA/turbo packages don't seem to affect the dyno chart at all (just scaling it up/down). I would be very surprised if it were possible to make an engine more efficient with different upgrades (in GT4).
 
fuel is good in GT4, i done 2 laps of fuji in my charger and 2 dots of fuel went down! in the skyline i dont think it went down at all
 
Well, the weight effect is definitely there....

Example:

I am running the Motegi Super Speedway event in the Formula GT series in B Spec. In that series, it is well-nigh impossible to gain a big advantage over the AI opponents. The whole race tends to become a game of leapfrog, and with the exception of the slower 5th and 6th place cars, its anybody's guess who will cross the line in 1st place out of the 1st four cars.

Well, I decided to try to find an advantage, I found it by reducing my fuel load. My B Spec driver was running pretty much the same lap times as everyone else, and if I filled the tank every time, he'd pit at the exact same time as everyone else, too. (due to tire wear). Well, I decided that I would only carry just enough fuel to get me to my next tire pit stop. Turns out that I could push the car just as hard as the others, not losing any time at all, but when the others went to pit, I still could go two to three more laps. Eventually, because I was getting a couple of extra full-speed laps in while the others pitted and then had a couple of laps to warm their tires, I gained enough distance to break the slipstream and stay out front.

So, the fuel load does matter. You will produce less tire wear with a lighter car driving at the same level.
 
neurokinetik
So, the fuel load does matter. You will produce less tire wear with a lighter car driving at the same level.
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Correctly.
Is the only way to win the Polyphony Digital Cup in b-spec mode.
 
Fuel load affects tire wear, eh? That's an interesting conclusion...I should get some super-softs and do some tests...

When I was b-specing the other 24Hour LeMans race, the b-spec pilot didn't get any better times during the period of low fuel versus high fuel. It's a rather long track though, and 3 sample laps on each half of the tank probably isn't enough. I ought to test it on MSLII.

P.S. I thought Intercoolers just enriched the fuel-air mixture by compressing the air more (via cooling) before it gets to the engine, hence giving more oxygen to the reaction. But I don't know a whole lot about that.
 
Jmac279
The only way to make more power is to add more fuel and more air ;)
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Not quite as simple as that... Most engines make the most power with an air/fuel ratio somewhere in the neighborhood of 12.5 to 1. If a particular engine is running richer than that, you can make more power by putting less fuel into the engine. You'll even get better fuel mileage, because the engine will be spitting out less unburned fuel.
 
"You can't add just one ..."

I know adding one (fuel or air) throws off the air/fuel mixture ... Thus why I said you can't add just one ...
 
Just to chime in... this weekend racing the El Capitan race in b-spec mode, my Evo 8 couldn't stay ahead of a certain TVR. [forgot which one] I was forced to reduced to limit the fuel to a half tank per pit stop. This seemed to improve my laps times enough to catch him on the last lap which was quite dramatic.
 
I'm almost positive there is a weight penalty associated with fuel. The Formula GT C'ship is one example. I've also seen lower lap times from my b-spec guy at a number of circuits when I pit for tires and half a tank or less. I can't say I've noticed any balance issues in A-spec associated with fuel level.
 
Interesting...


I did the El Capitan race in my Alfa 155 race car, and I noticed that it drank quite a bit of gas, and when it had 1/2 tank it was getting faster lap times. tires seemed to last longer too. I didn't do it in B-spec though, (fell asleep on lap 59 -woke up by biting my tongue)

About the charger drinking more gas than the skyline, Wha'd you expect? Its 7.2 liters vs 3.5 ish
 
menglan
About the charger drinking more gas than the skyline, Wha'd you expect? Its 7.2 liters vs 3.5 ish
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Skylines use a 2.6L engine...but anyway its more the fact its 1970 technology compared to 1990's technology... not really the displacment... and areodynamics play a huge role too...

If it was all about displacment then the new 6L Corvette wouldn't be getting 28mpg on the hwy...
 
great topic. ill keep this stuff in mind when i start doing the endurance races. From all these posts I gather it may be beneficial to run in A-spec mode when the tank is full to make sure you get ahead (cause we all know A-spec is faster) and B-spec when theres not so much in the tank.
 
by-tor
Thanks for the testing! Sounds to me like fuel usage is proportional to the amount of power being used
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This brings up another point to test: will a car with a good, flat power curve (like the Cizeta) be able to save fuel with short shifting? If you look at the Cizeta V16T's power curve, it produces over 500 hp accross a range of 2000 rpm (about 6000 to 8000), as opposed to having a sharp peak at 8000, which is the case on many other cars. So if fuel is directly related to the produced power and nothing else, that means short shifting with a car like the Cizeta won't do a lot of good unless your shift point is at like 6500 rpm. But late shifting, at 9200 rpm or so, would save more fuel than shifting at 8000 rpm to stay in the powerband. This would be backwards. Shouldn't short shifting save fuel no matter what the powerband is like?
 
In real life that is probably correct - naturally aspirated engines are probably running inefficiently after the power peak. However like I said before I would be surprised if GT4's fuel consumption model is that involved. Of course, nothing has been proven yet, so I will test this tomorrow if nobody else has by then (I am busy today).
 
Sven
To test if the latter is true, one would have to put various upgrades on the car that don't imply more fuel being injected into the cylinders, such as muffler/air cleaners or intercoolers, and see how those affect it. I have not done that yet.
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exhaust, intake, and intercoolers all result in more fuel being used if the same A/F ratio is maintained.
 
this question is in my head since I got the game. If the oil needs to be changed (in the game), can the engine of the car blows up at a point that you need to absolutely buy another car ??
 
There is a definite change in tire wear and lap times with the amount of fuel in the car and this is especialy helpfull on the 10-20 lap races where you need to pit 1-3 times depending on the tires but you dont need 2 tanks of gas. only put in a half and the tires will last longer and les weight= faster lap times :)
 
Sven
Shouldn't short shifting save fuel no matter what the powerband is like?
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If you test it in real life, short shifting doesn't save fuel ...

Using minimal throttle saves fuel ...
 
how do you tell in litres how much fuel you have used and how much you have left?.

Do all cars have the same capacity fuel tank?
 
"In real life that is probably correct - naturally aspirated engines are probably running inefficiently after the power peak."

Hence the term... "peak power" :) No reason to just make noise if you're not making power!
 
Small_Fryz
how do you tell in litres how much fuel you have used and how much you have left?.

Do all cars have the same capacity fuel tank?
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In real life, I go by the odometer and how much I put in the tank every time ...

So if I've travelled 500 km since my last fill and I'm getting low on fuel, I stop in at the gas station and fill the tank back up. If it takes 30 liters to fill it up ...

500/30 = 16.7 km/L (Metric)
100*30/500 = 6 L/100 km (Metric)
500*3.7854/(30*1.609344) = 39.2 MPG (US Gallons)
500*4.54628/(30*1.609344) = 47.1 MPG (Imperial Gallons)

Obviously, most cars have differing fuel capacities, otherwise those less efficient vehicles would be filling up constantly ...

As for in the game, I couldn't tell you ...
 
Small_Fryz
how do you tell in litres how much fuel you have used and how much you have left?.

Do all cars have the same capacity fuel tank?
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I don't think all cars have the same size fuel tank...for the tests I've done, I go into the pits to have it tell me exactly how much fuel I have left.

Jmac279
If you test it in real life, short shifting doesn't save fuel ...
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I thought it would...? Don't the cylinders fire less often at less RPM, hence using less fuel? Or do the fuel injectors just constantly spray fuel during the full combustion cycle? I'm no expert on how these engines work...
 
I tried testing short-shifting for a pair of tanks vs. not short-shifting and it made a suprising difference ... A very fun week, though :) A semi-welcome alternative to my typical "granny" driving ...

Anyways, flooring it to the speed limit every time and shifting at 5500 RPM for a couple tanks vs. flooring it to the speed limit and shifting at 4000 RPM, I actually saw a decrease in fuel economy ... My fuel milage dropped by half for those 4 tanks (37 MPG to the 15-21 MPG) ...

I've also tried short-shifting with minimal throttle and I also usually see a slight decrease in fuel economy ... A large difference in economy in my GF's car by short-shifting (about 8 MPG less) ...

The best technique for fuel economy that I've found is to use as little throttle as possible and run the revs up until they stop increasing, then shift ... This is both quicker than minimal throttle + short shifting and more efficient in my experience ... I've only tried it in several cars, though (1988 Suzuki Swift, 1987 Nissan 200SX, 1990 Chevrolet Sprint, 2003 Pontiac Sunfire, 1991 Chevrolet Cavalier Z24) ... I gained a rediculous increase in fuel economy in the Sunfire vs. how my friend drives it ... He was always complaining about how he should've got a Honda because of the Sunfire's poor fuel economy, but I was able to get over 40 MPG over the course of a day ... He doesn't even drive fast and was only getting ~28 MPG, but he was shifting at 2500 RPM and using considerably more throttle than I did ... My GF only gets 42 MPG in her Sprint, but I average around 56 MPG (depends on how many hills I take ... it's weak, so hills kill efficiency) ...
 
Also, if burnt off fuel does indeed make the car lighter, then does it also affect the weight balance? This may be hard to test...
 
Sven
Also, if burnt off fuel does indeed make the car lighter, then does it also affect the weight balance? This may be hard to test...
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If PD did their homework, then yes, it should, if the fuel tank does not sit at the center of gravity in the car. I imagine that in many race cars, that the fuel cell is located such that large changes in weight distribution don't happen due to different fuel loads.
 

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