A simple redesign of the petrol engine could greatly increase its efficiency when idling or driving at low speeds, as well as reduce the harmful emissions it produces. UK researchers say the new design could even make petrol engines as efficient as diesel ones.
In most petrol engines, fuel is ignited by spark plugs inside cylinders which house the engine’s pistons. When the fuel burns, it produces rapidly expanding gases which force the piston to move within the cylinder. This movement is harnessed to power the car.
One important factor in the efficiency of an engine is the amount of fuel it uses when idling. But there is a limit on this: squirt in too little fuel and it becomes so dispersed within the cylinder that it cannot burn and the engine stalls.
Squirt and burn
Car makers have experimented with various ways round this, such as creating a smaller fuel clouds, and controlling the timing and movement of air within the cylinder so that the cloud reaches the spark plug without dispersing.
“Doing that inside the cylinder requires many complex changes to the engine and you are limited by the valve timing and other factors,” says Brian Knibb, an engineer at Knibb Gormezano and Partners in Derby, UK, who is part of the team hoping to commercialise a new approach.
The new idea is to have an additional, smaller chamber attached to the cylinder containing the spark plug. When the engine is idling, the fuel cloud is injected into this chamber and ignited.
A crucial part of the design is the chamber’s shape, which carefully controls the airflow inside, trapping the fuel cloud so that it cannot disperse as it is pushed towards the spark plug. The resultant hot gases then expand into the larger cylinder to drive the piston as usual.
The smaller chamber allows a smaller amount of fuel to burn well. It also allows more careful control of the way fuel is burnt, so it combusts more completely and produces fewer of the nitrogen oxides known as NOx that form when fuel burns inefficiently. The technique, which was invented by engineer Dan Merritt, whilst working at Coventry University, UK, is known as MUSIC – for Merritt Unthrottled Spark Ignition Combustion.
When the engine needs to do more work, the fuel injector sprays a longer pulse into the chamber, while at peak performance a second fuel injector provides an additional pulse to produce a larger fuel cloud and more power.
Knibb says it is relatively straightforward to modify existing engines to run in this way. “To produce MUSIC engines, a factory would simply need to change the cylinder head fitted to engines, leaving the cylinders and the rest unchanged.”
Merrit is now working with a company called Powertrain Technologies in Norfolk, UK, to test the idea on the road. The team is currently modifying a 2.0-litre Ford engine and hope to test it in a Ford Mondeo with the aim to make it 25% more fuel efficient. The project is partly supported by the Energy Saving Trust, a non-profit organisation promoting sustainable energy use and the car maker Ford.
The MUSIC design should make it possible to close the 25% efficiency gap between diesel and petrol engines, says Merrit, which is mostly due to greater efficiency when idling or travelling at low speed. Diesel engines achieve this because they are able to idle using smaller amounts of fuel.
“The MUSIC gasoline engine has now demonstrated part load efficiencies as good as the diesel engine, but at a lower cost,” says Merritt, adding that most vehicles on the road spend the greater part of their lives working at light load or idling.
Although the extra chamber could certainly improve efficiency at low loads, the question is at what cost, says Robert Kenny, who works on engine design at Queen’s University Belfast, UK. “Using two fuel injectors will certainly increase costs,” he points out, “and the thermal load on the neck between the cylinder and chamber could be a long-term issue.” The chamber arrangement will also reduce performance at full load, he says, “but the only way to judge the success of the design is to build and test it.”
“Most car manufacturers are working in some way on tackling the poor efficiency of petrol engines at low loads, typically by controlling fuel movement better,” says Kenny. “The technology is far from dead.” Piezoelectric fuel injectors able to fire faster and even injecting multiple clouds of fuel are being investigated by Mercedes, among others, he says.