It is well known that the basic mechanism of the piston engine has inherent drawbacks that can not be overcome, such as :
1- Around the point where the pressure developed by the burning of the air-fuel mixture is maximum (Top Dead Center, or TDC), the engine produces little or no torque, because the moment-arm is close to zero.
2- At mid-stroke, the moment-arm is maximum, but the pressure is about half-spent, and the friction force between the piston and cylinder wall is maximum.
3- Higher compression rates, which are desirable in terms of efficiency, can not be achieved in a gasoline fueled engine, because the heat of compression elevates the temperature above the point of auto-ignition. This causes premature explosion of the mixture, often before the piston reaches TDC. The negative torque subtracts power from the engine. Besides, detonation creates high power shock waves that damages the engine.
4- There is only one power stroke for every four strokes of the piston (typical Otto and Diesel Cycles) or two full revolutions of the crank-shaft. Roughly, usable torque is delivered in only 160 degrees for every 720 degrees of revolution, or 22%.
5- During idling or low power demand, all cylinders are compressing air or air-fuel mixtures and wasting energy.
To get around problem 3, additives must be mixed into the gasoline, elevating the “octane” content and increasing the cost of the fuel.
Problem 4 is mitigated by incorporating multiple cylinders in an engine. For instance, to obtain high power continuously during the rotation of the engine operating with the 4-stroke cycle, 12 cylinders are required. This results in a smooth running but highly complex and costly engine.
For this reason, four, six and less often eight cylinder engines equip most automobiles today.
Two-stroke engines, rarely used in automobiles and trucks, have the same problems as four- stroke ones, with figures shown in 4 being 320 and 44%, but with added problems of their own that can not be discussed in this short essay.
1- Around the point where the pressure developed by the burning of the air-fuel mixture is maximum (Top Dead Center, or TDC), the engine produces little or no torque, because the moment-arm is close to zero.
2- At mid-stroke, the moment-arm is maximum, but the pressure is about half-spent, and the friction force between the piston and cylinder wall is maximum.
3- Higher compression rates, which are desirable in terms of efficiency, can not be achieved in a gasoline fueled engine, because the heat of compression elevates the temperature above the point of auto-ignition. This causes premature explosion of the mixture, often before the piston reaches TDC. The negative torque subtracts power from the engine. Besides, detonation creates high power shock waves that damages the engine.
4- There is only one power stroke for every four strokes of the piston (typical Otto and Diesel Cycles) or two full revolutions of the crank-shaft. Roughly, usable torque is delivered in only 160 degrees for every 720 degrees of revolution, or 22%.
5- During idling or low power demand, all cylinders are compressing air or air-fuel mixtures and wasting energy.
To get around problem 3, additives must be mixed into the gasoline, elevating the “octane” content and increasing the cost of the fuel.
Problem 4 is mitigated by incorporating multiple cylinders in an engine. For instance, to obtain high power continuously during the rotation of the engine operating with the 4-stroke cycle, 12 cylinders are required. This results in a smooth running but highly complex and costly engine.
For this reason, four, six and less often eight cylinder engines equip most automobiles today.
Two-stroke engines, rarely used in automobiles and trucks, have the same problems as four- stroke ones, with figures shown in 4 being 320 and 44%, but with added problems of their own that can not be discussed in this short essay.
|
|
Because of it's mechanism, this engine can only expand the gas by Notice that in the Lucas Cycle, because the compressor and the motor
the difference between the combustion chamber volume and the are independent the burnt gas is fully expanded down to atmospheric aspirated volume. The ratio between these volumes is 8 to 1. pressure. Using the same amount of fuel, the Lucas Cycle utilizes The pressure achieved at TDC is about 800 psi, afterwards it can more power than an Otto Cycle. This is represented in the graph above
only expand the gas down to 100 psi. by the green area.
At this point ( 5 ) the gas is vented to atmosphere.
This remaining volume of gas at 100 psi still contains significant The extra energy developed -at 100 psi- by the Lucas Cycle, as an
unused energy! example, would be enough to run pneumatic tools!
The diagrams are idealized cycles to make the concepts simpler to understand. For a more detailed analysis of the limitations of a 4 stroke engine please visit: http://mb-soft.com/public2/engine.html
http://www.iitg.ac.in/scifac/qip/public_html/cd_cell/chapters/uk_saha_internal_combustion_engine/qip-ice-06-valve%20timing%20diagrams.pdf
© 2011 Lucas Engines
the difference between the combustion chamber volume and the are independent the burnt gas is fully expanded down to atmospheric aspirated volume. The ratio between these volumes is 8 to 1. pressure. Using the same amount of fuel, the Lucas Cycle utilizes The pressure achieved at TDC is about 800 psi, afterwards it can more power than an Otto Cycle. This is represented in the graph above
only expand the gas down to 100 psi. by the green area.
At this point ( 5 ) the gas is vented to atmosphere.
This remaining volume of gas at 100 psi still contains significant The extra energy developed -at 100 psi- by the Lucas Cycle, as an
unused energy! example, would be enough to run pneumatic tools!
The diagrams are idealized cycles to make the concepts simpler to understand. For a more detailed analysis of the limitations of a 4 stroke engine please visit: http://mb-soft.com/public2/engine.html
http://www.iitg.ac.in/scifac/qip/public_html/cd_cell/chapters/uk_saha_internal_combustion_engine/qip-ice-06-valve%20timing%20diagrams.pdf
© 2011 Lucas Engines