Interview with Dave Morrison, Ricardo Motorsport, on the Ricardo / Engine Developments Diesel Le Mans Engine Project

The prospect of racing a diesel engine at Le Mans, with the aim of winning the race, first arose during the 2002 Autosport International show at the UK’s National Exhibition Centre. There, Ricardo Motorsport was approached by a team interested in such a project - but at that time it was unclear what the rules would be for a diesel engine. After Ricardo had discussions with the ACO about diesel engines, regulations for them were published at the end of 2002. Initially Ricardo considered three options: firstly its own design based on a 5-litre V10, then converting an existing production engine, before finally choosing to convert an existing race engine. Alan Lis asked Dave Morrison, Ricardo Motorsport Business Operations Manager, about the progress made so far and the future prospects for the project.

Why did you reject the idea of building your own engine?
“Although it was a conventional diesel, it would have had very high cylinder pressures and high torque, something like 1300 Nm at 2500rpm. That would have been a real challenge for the transmission and the engine itself would have been on the heavy side.”

When did you decide that converting an existing race engine was the way forward?
“In January 2003, when we started talking with Engine Developments Ltd about the possibility of converting their V10 engine to run on diesel fuel. We then started doing some theoretical modeling that showed that there were some possibilities. It’s a technically fascinating project and would be the first truly race optimised diesel engine. The diesels that are racing now are just conversions of road engines. This would throw all that out the window: we can forget about extreme durability, because the engine only has to last 30 hours so we can make a relatively lightweight structure. The bottom end of the Judd V10, with its bed-plate construction, is quite rigid, so we reckon it’s strong enough to take quite high pressures.”

At this point in time does an engine exist other than as a computer model?
“We’ve built a four-cylinder mule engine based on one bank of a Judd V8, to demonstrate the principle. That engine already has a very narrow valve angle, which suits a diesel application quite well. We designed the combustion chamber and Judd made it as a mule engine and, we very much hope, that will lead ultimately to a diesel V10, but the whole project depends on funding.”

What compression ratio would be required?
“Quite a low one by diesel standards. You couldn’t get away with the compression ratio we would use for racing on a road engine because of the cold start requirement. On a race engine you want to run as a low a compression ratio as you can get away with, to restrict the cylinder pressures.”

What sort of torque output is possible?
“With the engine we designed about three years ago, what I would call a pure racing diesel in terms of very high cylinder pressures, we were looking at very high torques, around 1200Nm. The new engine would generate a lot less torque although it would be more than a comparable turbocharged gasoline LMP900 engine, but not by a lot.

“The torque curve would have a different shape, because of the different characteristics of a diesel engine. There would be more torque at the bottom end, which means that driveability and pulling power out of corners will be much better than a gasoline engine. We have even been debating how many gears would be needed in the transmission to take full advantage of that. If you have a better torque curve shape maybe you don’t need so many gear changes. Less gear changes means a little bit of time saved on a lap.”

What would maximum rpm be?
“Fairly high for a diesel. A current diesel engine for a road car would make peak power at around 4500rpm, we are targeting a bit more than that. We’ve run diesels to 6000 before. Our speed will be high for a diesel engine but not for a gasoline engine.”

Do you have any projections on how fuel efficient the engine might be?
“Yes, we obviously done some modelling of this. We’ve predicted how many extra laps we would get. It’s no secret that the Audis with the DI engines were getting another 1-2 laps over the competition at Le Mans in 2003.”

In 2003 with the smaller ACO air restrictors the Audis were able to do 15 laps on a tank of fuel. Would the diesel engine be able to match that?
“It would be able to do more laps, perhaps 1-2 more. The initial calculations that we have done suggest that it’s enough of an increase to make it worthwhile. A racing diesel engine would be up against the same sort of problems as a racing gasoline engine. The principle that a diesel engine would be more reliable is not as valid in a racing application.”

What other changes than a new cylinder head design would be needed for a diesel racing engine?
“You would need new pistons for the changed compression ratio and a slightly taller block to accommodate them, because a modern day gasoline piston has very little skirt provision and a shallow crown, so there would not be enough room for a bowl. The pistons would have to be 20-25mm taller to make room for the bowl in the crown and that means a heavier piston. Some of the weight disadvantage could be overcome using compromises that have found their way into other race engines, such as two piston rings instead of the usual three.

“The aim would be to keep as much of the original engine design as possible. Pretty much all of the new parts would be at the top end of the engine. It’s the same process as converting a road engine from petrol to diesel. You might change the specification and size of the main bearings slightly, but typically you would try not to change the bottom end at all.

“The location of the injector would also be different because in the ideal diesel combustion chamber you want the injector positioned centrally and vertically, above a perfectly round bowl in the centre of the piston crown. That’s sometimes not achievable, but on most road engines, in most cases, it can be done. In a race engine it may be necessary to have more of a compromise. Packaging the injector is not easy because it’s quite a bulky item compared with a spark plug. Also it would have to withstand a high clamping because of the high cylinder pressure it would be working under. In an aluminium cylinder head, as you have on a race engine, the injector would need to be clamped down pretty firmly. There’s a bit of a design challenge in that area.”

Is the fuel feed strategy for a race engine any different to a road engine?
“The basic principle would be the same. You would be injecting the diesel fuel into air at a very high air:fuel ratio, otherwise there would be too much smoke. You can’t burn diesel like you would gasoline. You can’t ignite a homogenous mixture with a spark for example. If you ran the same air:fuel ratio as a petrol engine, there would be clouds of black smoke. A diesel engine typically runs an air:fuel ratio from 20:1 to 40:1.

“The control strategy for diesel fuel injection on a road car is quite complex. With modern electronic control and common rail injection, where you have a separate pump with a pressurised rail, you open the injector electronically to admit as much fuel as you want. All of the control is electronic so you can achieve multiple injection, special injection strategies for cold starting, for high performance, whatever you want. For racing the control strategy would actually be a bit easier, but not quite so challenging.”

Would a racing diesel engine not have been possible before electronic fuel injection?
“It would have needed a different approach. Boost management is the biggest challenge, to ensure that you’re getting the right amount of air into the engine.”

How are you addressing the boost issue?
“Ricardo has done a lot of research on two-stage boosting for research purposes in road engines. You are looking at very high boost and very high engine performance but it’s complicated and tends to add weight, so we’d rather not do that for a race engine. Because we’re operating at relatively low pressures we think we can probably just use single stage boosting. We’re talking to turbocharger manufacturers (that we have very close contacts with) to supply the technology to do that. It doesn’t have to be that complex.”

How much weight would a diesel cylinder head and turbocharger system add to the existing Judd engine?
“That’s a key question. Undoubtedly the diesel engine would be heavier: a common rail injector is a large and heavy steel item that has to withstand pressures of up to 1600 bar, so its has to be strong and heavy. Then you have ten of them sitting on top of the engine, not in an ideal position. We haven’t done the calculations yet, but I don’t anticipate it adding up to a big weight penalty. Not as a big a penalty as converting an existing road going V10 (one that’s designed for a service life of a quarter of a million miles) to a race engine.

“Of course racing cars are typically built underweight and ballasted up to the minimum limit. I would think that the additional weight of the diesel engine could be allowed for in reduced ballast.

“It’s a fascinating project I really hope we can do it….”