The mighty group C cars (and a bit of IMSA)

[langwadt]

I would like to see rotary engines back in racing.
They would fit perfectly in the fuel-flow-restriction-era of LMP now.

I would like to go even further and change the formula to energy-flow-limitation.

How would they fit perfectly?, don't think I've ever heard them described as fuel efficient

[Blanchimont]

In this paper( http://www.rotaryeng.net/Mazda_R26B_US.pdf ) the 1991 Mazda engine is quoted with 286g/kwh @~350kW output in efficiency mode at 6000rpm. What could be achieved 24 years later? Probably not the 220g/kWh of the Prius, but maybe 240/250?

[langwadt]

In this paper( http://www.rotaryeng.net/Mazda_R26B_US.pdf ) the 1991 Mazda engine is quoted with 286g/kwh @~350kW output in efficiency mode at 6000rpm. What could be achieved 24 years later? Probably not the 220g/kWh of the Prius, but maybe 240/250?

http://www.f1technical.net/forum/viewtopic.php?p=387327

if rotaries couldn't keep up with regular engines in terms of efficiency 24 years ago why should they now?

[bill shoe]

I'm a big fan of rotaries and I have huge respect for Mazda because they took a crackpot engine and made it the only piston alternative in the first century of automotive powertrains.

Wankel rotaries will never be high efficiency engines, at least not when compared to a four-stroke piston engine with similar money and engineering effort put into them. Rotaries have large amounts of combustion-chamber surface areas, especially when comparing the initial combustion phase of a wankle vs piston engine. This loses combustion heat and reduces efficiency. They can, sometimes, sort-of, have less friction than a piston engine. But you can take the piston engine and go to a larger displacement at lower revs and get the same thing.

Having said that, about four years ago I got to sit in the actual LeMans winning Mazda. It still has the same engine with the same parts that won the race (after the race it was taken apart, photographed, and reassembled). When the big four-rotor engine starts up and idles it has a big manly sound like nothing else on earth. Too much respect to put into words.

[Pedrolito]

There is a wonderful family pics on wikipedia.
From left to right : An XJR-9, three XJR-12s, another XJR-9, two XJR-11s, an XJR-10, an XJR-6, and an XJR-5.


Later, I will try to make a post with detail pics of the XJR from 5 to 14 (the Jaguar cars during the group C era).

[Blanchimont]

In this paper( http://www.rotaryeng.net/Mazda_R26B_US.pdf ) the 1991 Mazda engine is quoted with 286g/kwh @~350kW output in efficiency mode at 6000rpm. What could be achieved 24 years later? Probably not the 220g/kWh of the Prius, but maybe 240/250?

http://www.f1technical.net/forum/viewtopic.php?p=387327

if rotaries couldn't keep up with regular engines in terms of efficiency 24 years ago why should they now?

That's pretty much what i said. 240/250 > 220

But let's imagine a rule set that doesn't limit maximum fuel flow and engine weight. Say the rotary engine's bsfc is 10% worse than the best piston engine, it therefore would have to carry 10% more fuel to achieve the same power output. These 10% could be about 80l*0,75kg/l*0,1 = 6kg for a tank size of 80l, without a doubt, this makes the car slower. But what if the engine itself is 10kg lighter than the piston one?

[-Felix-]

I was astonished by the weight quoted in the paper.. 180kg für that small engine? quite a big lump in my eyes.. That's about the same weight as the biturbo V8 in the Sauber Mercedes..

[MadMatt]

I wanted to bring that thread back because looking at some old IMSA videos made me realize just how things are changing. If we look at the 1980s the guys tried to feed the rear diffuser as much as they could because they could make it start let's say half way between the front and rear wheels (Porsche 956). Then they tried to make the air from the front diffuser they created escape on the side (Toyota Eagle, Mazda RX-792P for example) with great success. But then they went back to fully closed sides (Jaguar XJR-14, Toyota TS010). And now with front wings and such, they are trying again to exhaust this air from the side or the top (Audi R8, and all modern LMP1 cars).

I am surprised by the constant change in philosophy to exhaust the air at the front, but also how they managed to produce enough front downforce to balance the huge bi-plan multi-elements rear wings they had on the Toyota TS010 for example. What did they have under the front of that car to balance the rear? I never found a picture of the underside.

[MadMatt]

For those interested by IMSA racing, you probably know these 2 guys:



ESPN commentators David Hobbs (ex sportscar racing driver on the left) and Bob Varsha (on the right) commentated a lot of IMSA races (for sure you will recognize Varsha's voice when you hear it).

You can watch a lot of IMSA races on youtube if you use for example "Imsa 1992" or "Imsa 1991" as search words.

Have fun!

[machin]

... but also how they managed to produce enough front downforce to balance the huge bi-plan multi-elements rear wings they had on the Toyota TS010 for example. What did they have under the front of that car to balance the rear?

No clue here either...

[VARIANT | one]

I am surprised by the constant change in philosophy to exhaust the air at the front, but also how they managed to produce enough front downforce to balance the huge bi-plan multi-elements rear wings they had on the Toyota TS010 for example. What did they have under the front of that car to balance the rear? I never found a picture of the underside.

I'd be guessing one or a few of the things here:

1.) Even though the wing is at the rear, the lower surface of the lower wing plane creates a negative pressure area behind the car "driving" the tunnels to pump harder and keep flows attached, whose suction peak is at the area just before they start to ramp upwards (near the forward-middle portion of the car). Costas Los said the Allard J2X-GTP would bias downforce forward by adding angle to lower wing plane, and the upper plane was necessary to balance it back to the rear.

2.) The cars simply moved their CG rearward somewhat to match the CP bias. Not so sure though with the radiators still in the front of the TS010 in particular. Everyone else moved them midships.

3.) Toyota caught on to running more rake like F1 cars do today, which moves the underbody suction peak forward, and/or got a similar effect with those fences around the car, which shed a vortex and emulate sliding skirts (which notoriously brought the CPs foward in F1 cars of their day).

4.) They simply ran with a higher rear downforce bias, which is actually acceptable (the car will simply always aero understeer at the limit) but still advantageous on some circuits where you need to put the power down. Note how only 31-33% of the downforce is on the front tires here: http://www.mulsannescorner.com/aerodata ... p3593.html

No. 4 could be a good bit of it. A lot of these cars were clamoring for front downforce when they were neutered of all the funtime aero madness. Cars like the Peugeot 905B Evo2, Allard J2X-GTP, Jaguar XJR-14, the unfinished Nissan NPT-93 all had concerted front wing/diffuser solutions going on. It would have only been a matter of time until they started to evolve pontoon fenders and boat prows to scavenge front aero down the sides of the car like we see today.

[wesley123]

I don't think they ever balanced it out properly

[VARIANT | one]

I don't think they ever balanced it out properly

Well, John Iley said they could run a 43% front bias if they wanted to. On a ~45/55 car, that'd be right in the pocket as the CP should be ~50 mm behind the CG (though I suspect the J2X-GTP was more like 40/60 with so much of it's mechanicals, and an unstressed motor, backed up on the rear axle). The nose of that particular car was an extreme step forward however, and simply ahead of it's time. =D>

I think with modern P1 cars, you've got the opposite going on, trying to push more mass towards the front. Front aero is more open to development than the rear (templated diffusers, dinky wings), and so much of modern prototype racing is about spending the least amount of time in the pits, so optimizing tire wear is probably driving the design of the cars. Wide tires at the front are indicative of this. I wouldn't be surprised if the cars with front MGUs like the R18 and 919 are running ~50/50 weight distributions to divide the wear out evenly on similarly sized rubber. Brake tech plays into this too, because if you can run 24 hours punishing the front discs and calipers without having to change them, the above becomes an option.

[VARIANT | one]

But let's imagine a rule set that doesn't limit maximum fuel flow and engine weight. Say the rotary engine's bsfc is 10% worse than the best piston engine, it therefore would have to carry 10% more fuel to achieve the same power output. These 10% could be about 80l*0,75kg/l*0,1 = 6kg for a tank size of 80l, without a doubt, this makes the car slower. But what if the engine itself is 10kg lighter than the piston one?

But that's assuming you can make a rotary lighter than a piston motor. It's an oft parroted "given" that Wankels are inherently light, but in motorsport (or high performance motorcycles, for that matter) where piston engines are far more optimized mass-wise, are they? Even from a similar era, the infamous 4-rotor 2.6 liter R26B weighed in at 180 kg. where a 5.0 liter Judd GV5-S2 tipped the scales at 130 kg. and revved higher to boot. 3.0 liter Formula 1 V10s got down to ~80 kg. A 3.0 liter V8 Radical Macroblock is 88 kg.

And that doesn't take into account the fact that Wankels cannot function as a stressed member (gander the engine bays of the RX-792P or 787B) as the chambers distort under beaming loads, and thus take on the mass of the necessary subframimg around them.

Add on top of that, there's more required radiator cross sectional area mucking with your critical aero, and the aforementioned fuel consumption issue in a sport where fuel consumption is paramount, so I'm fairly certain they'd not make the cut given more open engine regulations. Diesels wouldn't make the cut either. Ulrich Baretzky said that where they are with direct injection gasoline, they'd win out in a pure parity regs scenario.

[JJR]

... but also how they managed to produce enough front downforce to balance the huge bi-plan multi-elements rear wings they had on the Toyota TS010 for example. What did they have under the front of that car to balance the rear?

No clue here either...

http://www.dailysportscar.com/wp-conten ... 010_02.jpg

I think they used rear lower biplane wing to produce front downforce because it ehance rear diffuser performance as it starts closer to front axle than rear axle. And also with front louvres about front wheels.
And finally there are aero data on mulsannescorner.com page for TS010:

High downforce sprint configuration:
Downforce:
5061 lbs. @ 150 mph, with 830 lbs. of drag
7288 lbs. @ 180 mph, with 1195 lbs. of drag
8998 lbs. @ 200 mph, with 1475 lbs. of drag

Aero. Balance @ 200 mph:
F: 3869 lbs. (43.0%)
R: 5129 lbs.

Lift-to-drag ratio: 6.1:1

Low-drag sprint configuration (Monza):

Downforce:
4049 lbs. @ 150 mph, with 686 lbs. of drag
5830 lbs. @ 180 mph, with 988 lbs. of drag
7198 lbs. @ 200 mph, with 1220 lbs. of drag

Aero. Balance @ 200 mph:
F: 2951 lbs. (41.0%)
R: 4247 lbs.

Lift-to-drag ratio: 5.9:1

Le Mans:

Downforce:
3263 lbs. @ 150 mph, with 563 lbs. of drag
4698 lbs. @ 180 mph, with 810 lbs. of drag
5800 lbs. @ 200 mph, with 1000 lbs. of drag

Aero. Balance @ 200 mph:
F: 2320 lbs. (40.0%)
R: 3480 lbs.

Lift-to-drag ratio: 5.8:1

So we can see that car has enough front downforce - aero ballance 40-43% at front .