A higher compression ratio permit the same combustion temperature to be reached with less fuel, while giving a longer expansion cycle . This creates more mechanical power output and lowers the exhaust temperature.
What is the relation between compression ratio and efficiency of Otto cycle?
1. an ideal thermodynamic combustion cycle, as follows: a compression at constant entropy; a constant-volume heat transfer to the system; an expansion at constant entropy; and a constant-volume heat transfer from the system. The thermal efficiency of the ideal Otto cycle increases with an increasing compression ratio .
What is the effect of compression ratio on efficiency?
The thermal efficiency increases and the brake specific fuel consumption decreases with the increase of compression ratio. The higher compression ratio results in higher in-cylinder pressure and higher heat release rate as well as lower ignition delay.
What effect does increasing the compression ratio R have on the efficiency of the Otto cycle?
3. For a given specific-heat ratio , thermal efficiency increases dramatically with increasing compression ratio at low values of r. Beyond r = 10, however, thermal efficiency curves flatten out, and the benefit of operating at higher and higher compression ratios lessens.
What is the compression ratio of Otto cycle?
In the ideal Otto cycle the fuel-air mixture is introduced during the induction stroke and compressed to a much lower compression ratio ( around 8:1 ) and is then ignited by a spark. The combustion results in a sudden jump in pressure while the volume remains essentially constant.
Does higher compression mean more torque?
Increase in compression ratio induces greater turning effect on the cylinder crank. That means that the engine is giving more push on the piston, and more torque is generated .
What happens if compression ratio is too high?
(1) With high compression ratio, they operate at higher pressures and offer a cleaner and complete burn of fuel leading to better mileages too . However, it’s not ALL good. The higher pressure requires stronger build of the engine (in some cases heavier). Chances of knocking in the engine are higher.
What is K in Otto cycle?
Thermal Efficiency Derivation
A typical value for an air/fuel mixture is k = 1.35 .
Which cycle is more efficient Otto or Diesel?
6 Diesel Cycle
Although for a given compression ratio the Otto cycle has higher efficiency, because the Diesel engine can be operated to higher compression ratio, the engine can actually have higher efficiency than an Otto cycle when both are operated at compression ratios that might be achieved in practice.
What is the efficiency of Carnot cycle?
It is well known that the Carnot cycle efficiency ( η thermal = 1 − T L T H ) is maximized with the highest possible heat source temperature T H and the lowest possible heat sink temperature T L .
What is ideal cycle efficiency?
Thus, the efficiency increases with the compression ratio. However the compression ratio of Otto cycle engines is limited by the need to prevent the uncontrolled combustion known as knocking. Modern engines have compression ratios in the range 8 to 11, resulting in ideal cycle efficiencies of 56% to 61% .
Why is the compression ratio of an Otto cycle limited?
Autoignition – Limit for Compression Ratio
The unburned mixture may autoignite by detonating from pressure and heat alone , rather than ignite from the spark plug at exactly the right time. The engine knocking can be reduced by using high-octane fuel, which increases the gasoline’s resistance to autoignition.
What is the formula for compression RK ratio of the Otto cycle?
Explanation: This is the expression for efficiency of Otto cycle and rk= compression ratio=Vmax/Vmin.
When the mixture is lean efficiency is less?
Explanation: Power output is directly proportional to mixture, so when the mixture is lean, the power output is less and vice-versa.
What is Otto cycle and Diesel cycle?
Otto cycle is used for petrol or spark ignition engine while diesel cycle is used for diesel or compression ignition engine . The main difference between Otto cycle and Diesel cycle is that in Otto cycle heat addition takes place at constant volume and in diesel cycle heat addition takes places at constant pressure.
