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Source: Race Engine Technology

Publication Date: 17^th July 2007

John Judd’s company Engine Developments Ltd (EDL) powered the runner up at last year’s Le Mans race. That Pescarolo gasoline LMP1 car with its naturally aspirated 5.0 litre V10 engine finished just four laps adrift of the winning Audi V12 5.5 litre turbo-diesel after a full 24 hours of racing. For this year EDL has an enhanced 5.5 litre V10 and at the same time it is jointly developing with Ricardo a turbo-diesel derivative, to be able to fight Audi in future seasons like with like. John Judd Senior can therefore see the diesel picture from both sides, which is why I asked him for his views on it.

I started the conversation by referring to diesel restrictor sizes, currently some 50% larger than for the comparable gasoline engine. If you developed a gasoline LMP1 engine around an intake air restrictor area 50% greater than is currently permitted, I think it is fair to assume that you would have around 50% more top end power - something like 900 bhp?

"Yes, on top end power, with a petrol engine the horsepower is more or less proportional to the restrictor area." So why are the Audi and Peugeot turbo-diesel V12s, having that 50% intake area advantage, nowhere near 900 bhp?

"Because the diesel cannot use all of the air. With a petrol engine, with port injection or carburettors, you can essentially burn all of the oxygen. With a petrol engine horsepower is governed by the mass flow of oxygen - to all intents and purposes that is what counts.

"In fact by running the engine slightly rich of stoichiometric (Lambda = 1.0: the theoretical correct amount of fuel to combine with all the available oxygen) you make the most power. If you are running at about Lambda 0.92, you will draw out the oxygen molecules and combine them all with the fuel molecules in the time available for combustion.

"So you run richer than stoichiometric, at about Lambda 0.92. If you lean off from that, say to Lambda 0.94 or 0.95, you will get better fuel consumption but you start losing power. I would imagine this is pretty universal across this type of petrol racing engine.

"By contrast, the diesel, due to the nature of the fuel and the combustion process, it is not able to use all of the available oxygen. Unless you use a lot of excess oxygen it will smoke. You can get more power out of a diesel simply by chucking more fuel at it but it will smoke. If you have been to a tractor pull, you’ll have seen the mushroom clouds of smoke. So diesels are smoke limited.

"Petrol is volatile and evaporates - if you put petrol in a saucer it will steadily disappear, whereas diesel fuel will stay there forever. You need very high pressure to atomise diesel - nowadays they are looking to 2000 bar. The higher the pressure the smaller the droplet size, hence the bigger the surface area presented by the fuel in the chamber.

"Diesel has improved out of sight over the last few years in road cars, mainly due to improvement in injection technology. Pressure has increased tenfold, which has improved the ability of the fuel to mix with the air. So they are getting better air utilisation but it is still essentially an imperfect process.

"So in the case of a petrol engine there is a direct relationship between the amount of power that you can get and how much air you have. If you increase airflow by 50% you will increase power by about 50%. In the case of a diesel it is a case of how much smoke you are prepared to accept."

We are saying that the diesel needs the excess air to avoid visible smoke - so if that wasn’t an issue, you could get 900 bhp?

"I don’t know about 900. So far as I am aware nobody really knows - it is only of academic interest, with no practical value. It is not just the smoke that you have to worry about it is also the gumming up of the piston rings, things like that. If you ever see inside a diesel engine you will see how dirty it is, compared to a petrol engine.

"There are a lot of practical problems with diesel, for example the unburned carbon particles will form a hard layer on the side of the ring lands, which can lead to piston seizure. Diesel engine development is difficult and very specialised."

So what percentage air restrictor advantage for the diesel would be fair?

"I don’t know. The ACO doesn’t know - nobody really knows. Right now you have almost anti-diesel hysteria, with people saying that you can’t compete with a diesel engine. I don’t doubt that with the diesel particulate filter cleaning up the exhaust, the Audi can run a lap of Le mans with a fair amount more power than we have, without producing visible smoke. But I cannot say for how long they would be able to continue to do that...

"It all comes down to how good a job they have done on the combustion system. I don’t know how that has worked out for them. It is my suspicion that they have got a useful amount more power than we have got but I don’t see that as the whole reason for their race performance as having been so good. Their race performance was good before they had a diesel engine!"

You mention the power advantage over one lap but over a 24 hour race distance...

"I don’t know. Over a race distance they certainly have as much power as we have and they can get more from the engine. Diesel technology is evolving all of the time. Diesel road engines have become so much better over the last 10 years in a way that petrol engines haven’t. There has been an evolution in the technology of the diesel engine and I think that process of improvement will continue, although not so rapidly as we saw in the transfer from mechanical injection to high pressure common rail.

"I think diesel technology will continue to evolve and it is impossible for me to say what point Audi has reached. Obviously for them the Le Mans engine is a flagship project, so it has to be the best thing they have in the company.

"You also have to consider the fact that the fuel they run is not average pump diesel. It is formulated for its purpose: you have to consider that a bit of an advantage."

Also they have almost 3.0 bar boost: that must help the overall power curve?

"They can’t work a diesel at high rpm. There is a delay between when they inject the fuel and when it burns and that is an almost constant figure so as rpm increases you get to a point at which there isn’t time to burn the fuel properly. So they have to go down the route of low rpm and high boost pressure to get the power.

"Let us say, for sake of argument, they have 650 bhp at 4500 rpm, which is a bit more power than we have got and we achieve our maximum power at 7000-7500 rpm. To achieve maximum power at 4500 rpm they have much higher mep and, due to the nature of the compression-ignition process, much higher peak combustion pressure as well. Our peak combustion pressure is around about 85 bar, whereas theirs is probably around about 200 bar.

"You need an extremely strong engine to withstand that sort of pmax.

"Because the power is at relatively low speed, there is higher torque than we have but engine torque doesn’t really matter: the only thing that counts is tractive effort. It doesn’t matter what speed the engine is running at and what torque figure it is giving, that will all wash out through the gear ratios: what counts is the tractive effort at the rear wheels and that is a product of the horsepower the engine is making.

"One thing I think that is wrong with the regulations is that I don’t think the petrol engined cars should have to run the same car minimum weight as the diesels. The diesel is permitted to exploit certain engine advantages - they have got more power than we have - but they would have a very hard job to get their car below 925 kg. We could run less and to my mind the petrol engined cars are having to run the minimum weight that the diesels need. To my mind that is just as anomalous as last year giving the diesel the same tank capacity as petrol engined cars.

"I don’t like the fact that the ALMS rules are flexible but I do think it correct that they allow petrol-engined cars to run less weight. I think that 880 kg for the petrol LMP1 cars would be fairer for Le Mans and the ALMS."

Added to the database on 17^th July 2007

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Keywords: diesel horse power petrol gasoline