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Cars Under the skin: Why more pressure means cleaner petrol engines

02:20  03 december  2019
02:20  03 december  2019 Source:   autocar.co.uk

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The Skyactive-X petrol engine realises the Homogeneous Charge Compression Ignition-combustion It’s how sparkless diesels work and why they need a much higher compression ratio than petrol Lean burn simply means lots of air and less fuel. In a petrol engine , the perfect mixture to achieve

Under ideal conditions the common internal combustion engine burns the fuel/air mixture in the cylinder in an orderly and controlled fashion. Knocking is more or less unavoidable in diesel engines , where fuel is injected into highly compressed air towards the end of the compression stroke.

New, high-pressure injection systems are making petrol engines even cleaner © Autocar New, high-pressure injection systems are making petrol engines even cleaner Under the skin: Why more pressure means cleaner petrol engines © Autocar Under the skin: Why more pressure means cleaner petrol engines

Petrol direct injection (also known as gasoline direct injection, or GDI) is fast becoming the car manufacturer’s best friend. It gives better fuel economy and lower CO2 while improving low-end torque but, like most things, it’s not perfect. GDI engines generate more particulates in number (PN) and mass (PM) than a conventional port-injected engine.

One way to fix that is with higher-pressure fuel injection systems. Current GDI systems work at 200 bar but an industry first from Delphi Technologies is a 350-bar system. Delphi, Magneti Marelli and Bosch are all working on future 500-bar systems as well. First, though: what is it about GDI engines that makes them generate more particulates than a conventional port-injected ones? The fuel is identical, so what’s the difference?

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In diesel engine , unlike petrol engines , only air is injected at first, which is compressed. Just before piston reaching TDC, fuel injection starts and ignition happens because of the high pressure and also temperature situation inside the ignition chamber. The peak pressure is achieved at the moment

Gasoline( petrol ) engines operate within a narrow air/fuel ratio range of approximately 12:1 to 15:1, although some modern "lean-burn" technology engines The injected fuel in a diesel engine provides a less homogeneous air-fuel mixture in the cylinder, so combustion is less uniform and a cleaner burn

The traditional method injects a metered mist of fuel into each inlet port as pistons draw air in on the induction stroke. The fuel rushes through the tracts of the inlet manifold, evaporating into the rush of air as it goes, and arrives inside the cylinder as a lovely, evenly mixed (homogeneous) charge ready for ignition.

GDI engines, on the other hand, draw in fresh air only and inject the fuel directly into the combustion chamber as the air inside the cylinder is being compressed, just before combustion is due to happen. The inlet ports are designed to tumble the air into the cylinder so the fuel arrives in the middle of a tiny swirling storm that mixes the air and fuel together. But there’s still much less time for the droplets of fuel to evaporate than during that long journey the two make travelling into a port-injected engine, and incomplete evaporation of fuel droplets equals more particulates.

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MEP - Mean effective pressure indicates the utilization of engine volume. It depends upon the heat release pattern. Diesel engine got higher stroke length which means piston got more travel than a petrol engine and is helpful in developing higher torque [Torque=Force x distance].

Perhaps the most important is that it is inherently unstable and tends to explode under pressure , this makes it much more difficulty to store as it must be dissolved in acetone in an inert matrix, unlike say propane which can just be liquefied. This means that refueling would probably involve physically

Increasing fuel injection pressure to reduce the droplet size of the fuel achieves better evaporation in the shorter time allowed inside a GDI engine. Delphi claims that its industry-first 350-bar system reduces the size and number of particulates by up to 70% and that its forthcoming 500-bar system betters the former by a further 50%. But doing that isn’t as easy as it sounds – it’s not just a question of turning up the tap.

For a start, the higher the pressure, the greater the energy needed to produce it and that uses up a small proportion of the engine’s power and has a slight impact on fuel economy. It can also generate more noise and puts a much greater stress on all the injection components. New, more powerful injectors meter fuel more accurately, as well as coping with the extra pressure and detail changes to the fuel rail (tube) that feeds the injectors.

Delphi’s new design of injector pump delivers the extra pressure without increasing noise and minimising energy consumption. It also avoids increasing the load on the drive mechanism and subsequent costly modifications to the engine itself. The 350-bar system from Delphi is the first step and more – both 350 and 500 bar – should appear from other major suppliers in the next couple of years.

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Petrol can be ignited easily with a spark, hence does not require the kind of high compression ratios that are needed in a diesel engine . Please note that by changing the combustion concept it is possible for petrol engines to become humongous - jet engines on aircraft use refined petrol

Petrol engines are not meant for luxury cars; rather they are meant for all cars. Perhaps Ganesh is unaware of the fact that the smallest/cheapest cars are all petrol variants. Also , petrol burns cleaner than diesel which can be seen as black exhaust from diesel engines .

  Under the skin: Why more pressure means cleaner petrol engines © Provided by Autocar

Particulate filters like this one from Porsche are already being fitted to new cars to reduce particulate emissions leaving the engine by 90%. The filters are close-coupled and mounted very close to the engine so they ‘light off’ (get hot and start working) quickly. They also double as three-way catalytic converters already fitted to petrol-engine cars to clean up oxides of nitrogen (NOx), carbon monoxide (CO) and hydrocarbons (HC).

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