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Imagine a spark-ignition petrol engine with such a radical design that it has better fuel efficiency than diesel, and so lower CO2 emissions. Then consider an engine that produces zero particulates, and creates little NOx at up to mid-range power outputs. If high-power outputs are required, a catalytic converter is used to remove any offending NOx emissions.
Such an incredible product exists – it’s the Pure Burn engine, and we think it is the most important change to engine design in 130 years.
Although this engine was developed some years ago, the industry preferred to keep the diesel as the prime ‘fuel economy’ engine. This was understandable while government policy and taxation encouraged us to consider that diesel was the environmentally responsible option, as long as it remained within legislated limits.
Pure Burn is a unique design of cylinder head for petrol engines. The advantages are manifest: the engine offers very reliable starting and is extremely fuel-efficient – indeed, idling for 60 minutes produces the same amount of CO2 as a standard petrol engine would produce in four minutes. Very low levels of NOx are produced up to mid-range power outputs because of very fast combustion and flame dilution with excess air.
Unacceptable NOx production at higher power is fully treatable by a catalytic converter – a method that diesels cannot use. Although the Pure Burn engine is completely unthrottled in low- to mid-power ranges, at higher power it can make the exhaust oxygen-free by the use of automatic partial intake throttling. We think it’s the world’s only proven solution to address the diesel emissions crisis, while meeting all current CO2 targets.
Pure Burn achieves possibly the lowest fuel consumption and cleanest exhaust of any piston car engine by using a cleverly designed external combustion chamber, situated away from the engine cylinders. Unlike normal engines – which burn fuel directly over the piston – Pure Burn engines create a very highly-compressed film of rapidly spinning air/fuel mixture, pressed against the walls of a vortex tube by huge centrifugal force.
This unique method of further compressing and mixing the fuel and air results in very rapid and complete burning by reducing the distance between the gas molecules at the moment of spark ignition. The rapid combustion all but eliminates the formation of NOx.
Excess air is delivered after fuel injection stops, and is made to rotate on the inside of the burning vortex. As the flame expands radially inwards, violent mixing occurs with this excess air. Combustion is completed, eliminating exhaust pollutants, and the temperature in the chamber is reduced by the presence of this excess air, again suppressing NOx formation. The use of a volatile fuel such as petrol and the completeness of burning also mean that exhaust pollutants are minimised.
As the engine progresses towards full power, combustion temperature rises and the engine starts to create small quantities of NOx. This is addressed by engaging a partial throttle to bring the air-fuel ratio down to ‘normal’ levels close to 15:1, at which point no oxygen remains in the exhaust and any remaining NOx can be catalytically removed.
Critically, since nitrogen oxides and soot are absent, there is no need for particulate filters, or urea tanks.
From a thermodynamics point of view, the Pure Burn engine cycle is a textbook ‘constant volume engine cycle’, which is more efficient than the constant pressure diesel cycle. A proportion of post-combustion gases remain behind in the combustion chamber to act as a natural catalyst for the next cycle – this is naturally occurring exhaust gas recirculation (EGR). Traditional diesel engines need a range of pipes, valves and control circuitry to achieve this. By contrast, the Pure Burn engine achieves continuously variable and progressive EGR automatically without any control mechanisms or moving parts. The more fuel that is burnt, the greater EGR effect.
Unlike a traditional engine, the walls of the Pure Burn combustion chamber can be partially thermally insulated. Sufficient cooling is performed by the petrol vaporising as it is injected. So heat waste during the rapid combustion can be greatly reduced. So the maximum energy is extracted from the fuel during burning, and not wasted to heat up engine components, or dissipated through the radiator.
Our engine design is more than just a conceptual study. It has been developed over 30 years – initially at Coventry University. A prototype has been running on the test-bed under independent verification – with impressive results.
Tests so far have been on a four-stroke 2-litre engine, but design has recently started on a two-stroke variant to exploit the greater power density of two-stroke. This will have the option of using efficient roller bearings in a fuel-free crankcase environment. This could effectively double the fuel range of a standard petrol engine, while still being virtually pollution-free. It would find an ideal application in a hybrid powertrain.