Engine Cycles T Alrayyes
Introduction The cycle experienced in the cylinder of an internal combustion engine is very complex. The cycle in SI and diesel engine were discussed in detail in the previous chapter. Instead ideal cycles were assumed to make the process more manageable. Ideal cycle resemble true cycle but it is made of a lot of assumption.
WOT, naturally aspirated Engine
Assumption 1. Cylinder contains air during the total cycle and it is treated as ideal gas. 2. The specific heats and other physical and chemical properties remain unchanged during the cycle. 3. The real open cycle is changed into a closed cycle by assuming that the gases being exhausted are fed back into the intake system. 4. Instead of heat generation by combustion, heat is transformed from external heat source. 5. The process of heat removal in the exhaust gases is represented by heat transfer from the cycle to external heat sink Q out. 6. There is neither friction nor turbulence; all processes are assumed to be reversible. 7. The air is considered as an ideal gas
Ideal cycle Comparison between ideal and actual cycle
Air properties
WOT, naturally aspirated Engine
Assumption Actual engine processes are approximated with 4 ideal processes: 1. Isentropic during the compression and expansion stroke Assumed to be adiabatic and reversible process. Friction and heat transfer are ignored in the process 2. Constant pressure: during the intake and exhaust stroke The exhaust stroke is assumed to be constant at 1 atm. The inlet stroke will depend on the running conditions(wot, partially open and turbocharged) 3. Idealised combustion: is idealized by a constant-volume process (SI cycle), a constant-pressure process (CI cycle), or a combination of both (CIDual cycle). 1. exhaust blowby : is approximatedby a constant-volume process.
Otto Cycle 1 2 isentropic compression from V 1 to V 2 2 3 addition of heat Q 23 at constant volume 3 4 isentropic expansion to the original volume 4 1 rejection of heat Q 41 at constant volume
Otto Cycle
Otto cycle
Otto cycle η t Otto = 1 T T 4 1 1 T 1 T 2 T 3 1 T 2 Using the isentropic ideal gas relation and constant volume Where (V1/V2 = compression ratio r c ) and k is specific heat ratio Subsequently η t Otto = 1 1 c r k 1
Effect of compression ratio on thermal efficiency
Effect of r c and specific heat ratio
Diesel cycle Because of location and duration of combustion, diesel engine is based in constant pressure combustion rather than constant volume combustion. The rest of the cycle is identical to that of an SI engine
Diesel cycle
With the rearrangement, this can be shown to be equal, Where B = cutoff ratio Cutoff ratio is defined as the change in volume that occurs during combustion β = V 3 V 2 = v 3 v 2 = T 3 T 2
As the value of cut-off ratio increases ( heat addition is extended towards expansion) the efficiency is reduced due to additional heat required to compensate the expansion
Dual cycle Based on the equations of thermal efficiency of Otto and diesel engine. It can be concluded that Otto has a higher efficiency. Compression ignition would operate on the more efficient higher compression ratios, while constant-volume combustion of the Otto cycle would give higher efficiency for a given compression ratio. The modern high-speed CI engine accomplishes this in part by a simple operating change from early diesel engines. Instead of injecting the fuel late in the compression stroke near TDC, as was done in early engines, modern CI engines start to inject the fuel much earlier in the cycle, somewhere around 20 btdc Some of the combustion occurs almost at constant volume at TDC, much like the Otto cycle.
Dual cycle
Dual cycle
Cut-off ratio Pressure ratio, is defined as the rise in pressure during combustion, given as a ratio
Comparison of Otto, diesel and Dual Cycles: η t Otto = 1 1 r c k 1
Comparison of Otto, diesel and Dual Cycles: Direct comparison between the cycles based on the thermal efficiency equation will show that: The results suggest that the best combustion occur at constant volume and higher compression ratio
Comparison between Diesel and Otto cycle for the same compression ratio Thermal efficiency of the ideal Diesel cycle as a function of compression and cutoff ratios (k=1.4).
Comparison of Otto, diesel and Dual Cycles:
Comparison of Otto, diesel and Dual Cycles: s
Difference between Ideal cycle and real cycle Real cycle open with an open cycle and changing composition The gas inlet is different that outlet gas The mass flow rate is not the same (adding fuel after the induction, loss of mass during the cycle during to crevice and blowby) Treating the flow as air : Up to 7% is fuel After combustion the composition change Assuming an ideal gas with a constant specific volume Ideal gas is valid for low pressure but can deviate as the pressure increase Cp and Cv have high independence on temperature, can
Heat loss neglected During combustion lower peak temperature and pressure at the start of expansion Heat transfer during expansion and compression below ideal isentropic Cause a lower thermal efficiency. Perfect and Constant volume assumption Combustion starts btdc and continue atdc A quick but finite speed is desirable (steady pressure and force on the piston head causing a smooth engine cycle) A super sonic combustion would cause damage to the cycle Constant volume heat rejection It was assumed that exhaust blowby occur as the exhaust valve open due to pressure difference Exhaust blow by requires a finite time for that reason the exhaust valve must open 40 degree to 60 degree before bbdc
Valve timing Both exhaust and intake valve are not open exactly and BDC and TDC Intake valve is not closed until abdc due to flow restriction. Actual compression doesn t start until the valve is closed. Pressure is less than expected during combustion. Engine require finite time to actuate
Engine cycle at part throttling Part throttling create a flow restriction which causes a drop in inlet pressure. Fuel is also reduced to match the reduction in air. The more the throttle is close the higher the drop in pressure. This means that closing the throttle will reduce net work by reducing fuel (energy input Q in ) and increasing pumping loss
Supercharger and turbocharger Turbocharger has an adverse effect than a throttle Turbocharger will increase the inlet pressure above atmospheric pressure More fuel and air (more energy Q in ) Increase in air temperature in the intake stroke (compressive heating). Subsequently increase temperature at the compression stroke and the rest of the cycle. This might cause self ignition and knocking problems. Aftercooler might be used in some engines
The throttling and turbocharging effect on net work is captured through the following: Work done during the intake stroke is, Where V d is the displacement volume Work done on the exhaust stroke Total work is
Exhaust process The exhaust process contains two processes: The blowby: sudden drop in pressure large percentage of the gas leaves until pressure equalize Exhaust stroke: still some exhaust in the chamber and exhaust pushed by compression The temperature cool by expansion during the process. The temperature is initially cooled due to drop in pressure As the exhaust is pushed, a high kinetic energy is generated due high velocity flow This energy is transferd into enthalpy and temperature.
Initially drop to T 7, Although this expansion is not reversible, the ideal gas isentropic relationship between pressure and temperature serves as a good model to approximate exhaust temperature T7 in the hypothetical process
The state of the exhaust gas during the exhaust stroke is best approximated by a pressure of 1 Atm, a temperature of T7 and a specific volume shown at point 7.