Question
I understand that in diesel engines, the ignition in the cylinder is caused by the heat produced in compression of the air when the piston moves up and the fuel is injected. In gasoline engines, the ignition is caused by sparks once the air/fuel is mixed and compressed. Why Not use a spark to ignite the diesel engine combustion, or Why Not use compression methods (and do away with the spark) to ignite gasoline engines, as gasoline can also self combust like diesel when compressed?
I understand that in diesel engines, the ignition in the cylinder is caused by the heat produced in compression of the air when the piston moves up and the fuel is injected. In gasoline engines, the ignition is caused by sparks once the air/fuel is mixed and compressed. Why Not use a spark to ignite the diesel engine combustion, or Why Not use compression methods (and do away with the spark) to ignite gasoline engines, as gasoline can also self combust like diesel when compressed?
Answer
It is commonly known that a Diesel engine of the same capacity as its Petrol counterpart is more fuel efficient (approximately 10%). The main reasons why a diesel returns better economy is because of its ability to run very lean Air Fuel Ratios, better thermal efficiency aided by its higher Compression Ratio (CR) and significantly less pumping losses at part load due to the lack of a throttle valve. Diesel engines are not that fussy about the measures of fuel they receive, as long as they get some, they’ll burn it and produce useable power. Petrol on the other hand is far more choosy. If the Air Fuel Ratio (give or take a few ratios) isn’t around the stoichometric value then it really doesn’t want to burn (Stoichometric is the term that identifies the Air Fuel Ratio that offers the most complete burn resulting in the lowest emissions for the hottest flame. For unleaded petrol, it is 14.67:1, which is commonly rounded to 14.7:1. The stochiometric value for other fuels varies with their energy content.) Trying to run a petrol engine any leaner results in partially burnt fuel, unstable combustion and high Hydro Carbon (HC) and Carbon Monoxide (CO) emissions. Engineers for years have tried to combine the economy of a Diesel engine with the power of a Petrol Engine. There are two main ways of achieving better economy with a petrol engine. The first one is to get the engine to burn very lean mixtures (lean burn engine) and the other is to create a localised stoichometric cloud of mixture at the spark plug (stratified charge engine). The goal of the stratified engine is to run at Wide Open Throttle (WOT) and control the power in much the same as a Diesel by introducing varying amounts of fuel. Under light load conditions it is possible to run AFR’s as high as 60:1. The stratified charge is not a new concept, Ricardo were experimenting with the technology back in 1922. Early stratified engines used traditional carburettors along with a separate mixing chamber to mix the chemically correct AFR mixture which was then introduced into the ‘Clean air’ in the combustion chamber before ignition. Petrol engines also have an optimum timing window when the ignition should ignite the mixture. Too early and the engine will produce too many Oxides of Nitrogen (Nox) and advanced even earlier will begin to ‘Knock’, too late and you only get partial combustion and very high exhaust temperatures. The perfect ignition timing is the Minimum advance for Best Torque (MBT). Stratified charge engine make the timing of the ignition even more critical as the AFR at the spark plug changes as the cloud of chemically correct mixture passes through it. Careful consideration has to be given to the shape of the ramp on the piston as well as the injection angle, pressure and timing in order to coincide with optimum ignition timing. Sometimes throttling is needed at certain engine speeds in order to create the necessary air velocity to adequately mix the air and fuel. Diesel needs none of this.
It is commonly known that a Diesel engine of the same capacity as its Petrol counterpart is more fuel efficient (approximately 10%). The main reasons why a diesel returns better economy is because of its ability to run very lean Air Fuel Ratios, better thermal efficiency aided by its higher Compression Ratio (CR) and significantly less pumping losses at part load due to the lack of a throttle valve. Diesel engines are not that fussy about the measures of fuel they receive, as long as they get some, they’ll burn it and produce useable power. Petrol on the other hand is far more choosy. If the Air Fuel Ratio (give or take a few ratios) isn’t around the stoichometric value then it really doesn’t want to burn (Stoichometric is the term that identifies the Air Fuel Ratio that offers the most complete burn resulting in the lowest emissions for the hottest flame. For unleaded petrol, it is 14.67:1, which is commonly rounded to 14.7:1. The stochiometric value for other fuels varies with their energy content.) Trying to run a petrol engine any leaner results in partially burnt fuel, unstable combustion and high Hydro Carbon (HC) and Carbon Monoxide (CO) emissions. Engineers for years have tried to combine the economy of a Diesel engine with the power of a Petrol Engine. There are two main ways of achieving better economy with a petrol engine. The first one is to get the engine to burn very lean mixtures (lean burn engine) and the other is to create a localised stoichometric cloud of mixture at the spark plug (stratified charge engine). The goal of the stratified engine is to run at Wide Open Throttle (WOT) and control the power in much the same as a Diesel by introducing varying amounts of fuel. Under light load conditions it is possible to run AFR’s as high as 60:1. The stratified charge is not a new concept, Ricardo were experimenting with the technology back in 1922. Early stratified engines used traditional carburettors along with a separate mixing chamber to mix the chemically correct AFR mixture which was then introduced into the ‘Clean air’ in the combustion chamber before ignition. Petrol engines also have an optimum timing window when the ignition should ignite the mixture. Too early and the engine will produce too many Oxides of Nitrogen (Nox) and advanced even earlier will begin to ‘Knock’, too late and you only get partial combustion and very high exhaust temperatures. The perfect ignition timing is the Minimum advance for Best Torque (MBT). Stratified charge engine make the timing of the ignition even more critical as the AFR at the spark plug changes as the cloud of chemically correct mixture passes through it. Careful consideration has to be given to the shape of the ramp on the piston as well as the injection angle, pressure and timing in order to coincide with optimum ignition timing. Sometimes throttling is needed at certain engine speeds in order to create the necessary air velocity to adequately mix the air and fuel. Diesel needs none of this.
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