WEC Engine configuration vs regulations

[brutus]

Looking at the new WEC regulations, which establishes fuel flow limits, i was wondering how come manufacturers are using such different engine configurations? I would understand that if you had a displacement limit only, but when you establish a fuel flow limitation, shouldnt the most fuel efficient design prevail?

I have difficulty understanding toyota's choice for a NA v8. For one thing, i thought more cylinders equal less fuel efficiency, due to more heat losses in the combustion chamber and more friction losses. (more cylinders make sense with a displacement rule, because they yield more specific output).
And then there's the fact that turbos should be more efficient, since they use the exhaust gases that would otherwise be wasted. So why would anyone choose to go NA if they are restricting fuel flow? The Porsche architecture (V4 turbo) would seem to be the most efficient.

[NL_Fer]

Not only Toyota but also Mazda and Honda are building NA cars, because they believe it is the most effecient way. The TS40 ICE is not revving that much, 6000-7000 before shifting i believe. A Turbo also generates allot of heat, which is lost energy also.

If you keep the ignition and valve timing right, there shouldn't be that much lost energy in the exhaust. And most important, Toyota has build a very competative car this year, their approach works.

[Lycoming]

A Turbo also generates allot of heat, which is lost energy also.

But that heat is recaptured from the exhaust, so it's energy that would have been lost either way. More importantly, it increases cooling load by recapturing exhaust energy, as we saw from the F1 cars this year.

[NL_Fer]

I guess Toyota found some other way to reduce lost heat and pressure through the exhaust.

[machin]

I too was intrigued by this same question: on the road nearly all manufacturers seem to be switching to small capacity turbo engines to improve mileage... so why did Toyota go with a large capacity NA engine in a fuel efficiency formula? Well in my professional capacity I can get access to some real BSFC maps for various engines and what I find is that (already posted on the main WEC thread):-

A low compression, small capacity, turbo engine has smaller pumping losses at part throttle than a large capacity, high CR engine, resulting in more efficiency (lower BSFC) during low load running, however, the low compression means it is less efficient (higher BSFC) at full throttle.

A high compression ratio, large capacity, NA engine is more efficient (lower BSFC) at full throttle, due to the high Expansion Ratio that a high compression gives. However the big capacity high compression NA engine has higher pumping losses, and at part throttle (low load) the pumping losses become much bigger in relation to the output than a small capacity low compression engine, and therefore results in worse efficiency (higher BSFC).

Considering that the majority of driving on the road is conducted at part throttle it is not surprising that manufacturers are switching to small capacity turbo engines since these have better fuel economy in "normal" driving conditions, but still have the power for overtaking manoeuvres, even if the economy at full throttle is worse. However in a race situation, where a large proportion of the time is at full throttle, the choice is not so clear cut, which is why we see one manufacturer with a large capacity NA engine competing on pretty much equal terms with another manufacturer having a small capacity turbo engine.

[PorscheLMp1Fan]

Toyota's engine has some real cool stuff going on.

[machin]

Toyota's engine has some real cool stuff going on.

http://i.imgur.com/i1kQqnt.jpg

cool... Do you have the following pages?

[PorscheLMp1Fan]

cool... Do you have the following pages?

Thought I'd posted it

Edited it into the post.

[Pierce89]

Thanks Porschefan! That will make great reading for those on this forum that believe that small turbo motors are better than a larger NA motors in virtually all situations. Some here have an almost dogmatic view on these things and get overzealous in their small turbo vs large NA arguments. Unfortunately, I don't think this article would make one bit of difference to them. In fact, some people on this forum have gone so far as to claim that lack of budget is the ONLY reason Toyota didn't develop a turbo motor for this season.

[mike]

I would like to try and clarify the small engine turbo vs NA comparison

Basically efficiency is a product of 4 factors:
volumetric efficiency X thermal efficiency X mechanical efficiency X chemical efficiency

while fuel efficiency at WOT is a product of 3 factors
thermal efficiency X mechanical efficiency X chemical efficiency

-at WOT having a higher volumetric efficiency only means the engine has higher power density
-the only way to have higher fuel efficiency is to improve thermal/mechanical/chemical efficiencies

take an example 4 liter NA engine vs 2 liter with 1 bar of boost:
-assuming the knock limit is the same for both engines they will both have the same temperature at near TDC,
which means that the turbo engine a lower compression ratio which implies lower thermal efficiency
-apply this to WEC rules where max fuel flow rates are the same for both engines the NA engine will have better fuel consumption or high power with better efficiency
-what Toyota has done with their engines is that they improved the exhaust flow to have less exhaust gases in their cylinders, so engines can have even higher compression ratios to achieve better fuel economy
that being said, the turbo engine will have better weight and packaging advantages due to a higher power density

something interesting to consider is that engines usually have high mechanical as well as chemical efficencies (around 85% mechanical and 95% chemical) where as thermal efficiency are around 30 percent
turbo engines will have double the bearing load which will make it almost the same as NA engine with more cylinders only difference will be the wall friction, since blow bys are less of an issue in racing cars, the difference will be less significant if you were have better thermal efficiency.

[Cold Fussion]

What are the fuel rules for WEC? That image from PorscheFan above seems to suggest there is a spec fuel (at least for petrol), which isn't well suited to the Toyota engine?

[langwadt]

What are the fuel rules for WEC? That image from PorscheFan above seems to suggest there is a spec fuel (at least for petrol), which isn't well suited to the Toyota engine?

Shell is the sole supplier of fuel, afaict E20 for the petrols, B10 for the diesels

[zonk]

What are the fuel rules for WEC? That image from PorscheFan above seems to suggest there is a spec fuel (at least for petrol), which isn't well suited to the Toyota engine?

Shell is the sole supplier of fuel, afaict E20 for the petrols, B10 for the diesels

[youtube]http://www.youtube.com/watch?v=a4DPTmkk9jA[/youtube]

[PorscheLMp1Fan]

What are the fuel rules for WEC? That image from PorscheFan above seems to suggest there is a spec fuel (at least for petrol), which isn't well suited to the Toyota engine?

Toyota have stated that the fuel used was their 3rd choice fuel that was offered. (Not sure if that's in the article, haven't read it for a while). It was, however, Porsche's 1st choice. Maybe it was a compromise or maybe something else at play.