A new type of petrol engine is claimed to be as efficient as diesel.

It seems hard to imagine in these days of spiralling petrol prices that an invention that boosts greater efficiency would fail to spark significant interest among auto makers. But that, to date, is the case with a new prototype engine that is claimed to greatly improve fuel economy, particularly in urban driving.

The technology comes under the unlikely ancronym Music, which stands for Merritt Unthrottled Spark Ignition Combustion, an engine developed by Dr Dan Merritt of Coventry University.

By adopting diesel engine processes and incorporating them into a petrol engine, Merritt and his team say they have achieved a 20 per cent gain in efficiency, as well as a significant reduction in carbon and nitrogen emissions.

A diesel engine is more efficient than a petrol one because it sprays tiny droplets of fuel into very hot air. Diesel fuel will ignite on contact with the heat. A petrol engine has to create a correct mix of air and fuel, which is less efficient.

With a petrol engine, if you want half the fuel burned you have to halve the quantity of air, and the vehicle does this by using a throttle valve that obstructs the air flowing to the engine. Why is this inefficient? "However little fuel you burn in a petrol engine the temperature in the combustion chamber is very hot all the time because it is a chemically correct mixture, and the heat loss during the combustion process where the temperature reaches nearly 2,000° is high even at part load," Merritt explains.

"Whereas, in diesel, the temperature after mixing very quickly becomes much lower and therefore the heat loss is less. Also, when you are burning at a very high temperature you will oxidise nitrogen and this is an endothermic process, which means it removes heat from the combustion, it doesn't add to it because nitrogen is not a fuel. Also, whenever you have a closed or semi-closed throttle valve you have to draw in against a semi-closed valve, which is a pumping loss."

That was the challenge the team faced, and the only way to solve it effectively was to allow the spark to ignite the fuel-air mixture in such a way that air which does not contain fuel, does not mix with the air-fuel mixture. This process is know as 'stratification'.

The auto industry has tried stratification methods in the past and some of them succeeded partially. Honda did manage to produce an engine that was stratifying, but it did not work over the whole range and certainly none of them worked at the very lean end (1bar, 2bar, 3bar BMEP), which cars today use for urban driving.

Chamber music

The Music engine uses a separate combustion chamber into which most of the air mass is transferred. Fuel is injected into one end of the chamber and the spark plug is located at the opposite end. The chamber is designed to allow for fully-controlled air motion and thus managed stratification of the fuel and air.

This allows for a balanced mixture to be held around the spark plug while the mixture in the rest of the chamber can be lean or even have no fuel at all depending on the required load on the engine. The engine can run at global air-fuel ratios up to 100:1. This results in dramatic reductions in combustion temperatures and, coupled with a reduced surface area, a very significant reduction in heat loss.

"We managed to achieve [stratification] after a fairly long struggle and we have been at this on and off for almost 20 years," Merritt adds. "Music is the culmination of this process. We have been through a number of different designs and inventions, which did work up to a fashion, but not really suitable for mass production. They were a bit more expensive and a bit more complicated to sell.

"Music, which is the end of this process, is absolutely beautiful because it is very simple. It doesn't involve any moving parts, it is just a construction on the cylinder head. You can start the engine, idle it and go to full power without throttle.

"You get the full advantage of the thermo-efficiency of the diesel and in addition we found you can actually better the fuel efficiency of diesels because it has got some characteristics that diesel hasn't. For instance, the combustion is faster because gaseous petrol burns faster than droplets of diesel and the faster the combustion the more efficient the engine, basically."

The beauty of an external chamber, as used in Music, is that you can control the air movement in the chamber.

In a direct engine the air comes in through the valves and after that you really have little control over what happens to the air flow. It is possible to generate a bit of swirl by putting manifold inlet valves in certain directions, but it is much less precise than what Merritt has achieved with the transfer orifice. Here, the piston actually shoots the air into the compression chamber at the velocity you want it to be and in the direction that you want it to be.

In a swirl

"You are effectively giving a forward momentum to this swirling flow - firstly, it is a swirling flow and, secondly, you have to aim it to move forward like a screw thread," Merritt says. "So, air that has got in early finds its way towards the spark plug, which is at the other end, and air that comes later finds itself stuck in behind it.

"If you get fuel early you have a fuel-air mixture near the spark plug that can be ignited. If you then stop pushing fuel afterwards you have this air without fuel stuck behind so the two don't mix very well and the spark plug is able to ignite from small amounts of fuel in that way. It is a bit more complicated than that but in a simple way that is the best explanation."

The only additional control that you need on the engine is to start the fuel injection process at the right time. That is quite important because air movement through the orifice between the sealed end and the combustion chamber is not constant. "At the end of the compressor stroke, the velocity goes up because the pressure goes up all the time and also the mechanism of transfer is such that the pressure difference becomes bigger towards the end so that the velocity gets bigger towards the end," Merritt explains.

"So, if engine speed increases and you want to inject fuel into the air jet at a time when one gram of fuel is mixed with 15g of air, which is a function of the velocity, if the engine speed is higher you have to inject a bit earlier, because the high velocity starts earlier.

"At low velocity you have to inject late and at high velocity you have to inject earlier. While you inject you get this thermo kinetic mixture near the spark plug and if you continue to inject after that you generate lean mixtures and while the kinetic mixture begins to burn you can easily ignite the lean mixture behind it."

After the initial Music concept was tested on a single cylinder engine at Coventry University, Powertrain Technology took up the mantle and built it in a proper casting based on the Ford Mondeo engine, and the results there were even more impressive, although Merritt admits there is plenty of fine-tuning to be undertaken. All that remains now is for Merritt to find an OEM (original equipment maufacturer) to take the project on and deliver the efficiency benefits that the consumers will soon be crying out for.

"We are hoping that an OEM will take it up, but I must say that we have had very little contact with the German motor industry," Merritt says.

"The US is a route for this as this technology benefits the larger engines more than the smaller engine."

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