Engine Cycles Lecture
Outline In this lecture we will: Analyse actual air fuel engine cycle: -Stroke cycle -Stroke cycle Compare these cycles to air standard cycles
Actual Engine Cycle Although air standard cycles provide an effective method of estimating the efficiency of an engine and for comparison purposes between engines; There are a number of differences that exist between an actual engine cycle and an air standard cycle. These differences are clear when comparing the PV diagram from a real engine with that from an ideal cycle.
Actual -Stroke Cycle : BDC, start of the compression stroke. : Intake valve closed. Delay in closing the valve is to increase the volumetric efficiency. Actual compression doesn t start until the valve is closed. This reduces T and P predicted before combustion. 9
Actual -Stroke Cycle : Start of combustion. : TDC. -: Combustion process. Combustion requires a finite time to occur and so it is started before the TDC. Combustion continues at the start of the power stroke. 9
Actual -Stroke Cycle -: Actual expansion process. : Exhaust valve is opened. -: Main exhaust blowdown. The exhaust valve is opened before BDC since the blowdown process requires a finite time to take place. 9
Actual -Stroke Cycle -: Main exhaust process. : Intake valve opens. 9: TDC : Exhaust valve closes. -: Valve overlap period. This overlap period reduces the efficiency of the cycle but is imposed by the practical application requirements. 9
Actual -Stroke Cycle -: Induction process : BDC : Intake valve closes -: Exhaust and intake processes that form a loop representing pumping work. This reduces the efficiency of the cycle by a magnitude depending on the throttle settings. 9
Actual -Stroke Cycle For a wide-open throttle, pumping losses are minimal. While for partially-closed throttle, more pressure losses result in more drastic results. For a turbocharged or a supercharged engine, this loop is flipped upwards due to the high intake pressure. 9 9
Actual -Stroke Cycle : BDC, piston starts moving upwards -: gas exchange on scavenging : Transfer port is closed. -: Gases still escaping through exhaust port. : Exhaust port is closed
Actual -Stroke Cycle -: Actual effective compression process. : Start of the combustion process. : TDC -: Actual combustion process. -: Expansion process. : Exhaust port is opened. -: Exhaust blowdown.
Actual -Stroke Cycle : Transfer port is opened. -: Gas exchange process, both ports are opened. : Transfer port is closed. Loss of fresh charge results during this process.
Actual -Stroke Cycle The previous loop represents the cylinder itself. A pumping loop in the crankcase can be drawn as well. : BDC (this is the smallest volume in the crankcase). : Transfer port is closed. -: Gas transfer to the cylinder........9....
Actual -Stroke Cycle : Inlet port is opened. : Inlet port is closed. -: Induction process. : TDC (largest volume in crankcase) -: Charge is compressed in the crankcase. : Transfer port is opened. -: Transfer process........9....
Actual -Stroke Cycle
Actual Vs Ideal Cycles In addition to these differences, resulting from: The fact that the system is in reality an open air/fuel system (in contrast with a closed ideal air system). Finite heat release during combustion. Pumping loop loss due to work required to induce and expel gases. Other differences exist: Heat losses from and to the cylinder walls during the compression and expansion processes. Compression and expansion indexes are actually lower due the presence of the fuel and due to the mixture temperature.
Actual Vs Ideal Cycles Non-ideal combustion means that not all its chemical energy will be released. Engine friction which is produced throughout the engine cycle. Due to the combination of these factors, along with the differences discussed earlier, a considerable loss of efficiency is expected. As a rule of thumb, one can expect the actual thermal efficiency to be around. of that calculated theoretically.