The upper limit of Compression ratio in any engines will be knock limited. When the compression ratio is increased for a particular geometry engine, the cyclic variations in combustion tend to increase. The accepted limit of COV of IMEP is around 5 beyond which the combustion becomes unstable and tend to deteriorate, diminishing the performance and emission characteristics of the engine.So the upper limit of CR should be based on statistical analysis of COV.

Thanks for responding my query but I would welcome answers from other researchers also to explore

What should be the numerical limit for maximum compression ratio?

Apart from above response, what are the practical difficulties in achieving the numerical limits?

The upper limit of CR in diesel engines is determined primarily by:

a) Engine weight : higher peak pressure requires thicker walls to withstand mechanical stresses, adding to the weight and cost, and

b) The law of diminishing returns : the improvement in efficiency while increasing the CR from say 18-20 is lower than that of increasing it from say 14-16

The maximum value of CR I have come across is 25 : 1. Normally, the higher CRs are found in indirect injection engines.

Also, the CR is not limited by knock in diesel engine, as knocking tendency reduces with increase in CR due to shorter ignition delay and lower rate of pressure rise.

Thank You Saleel Ismail for responding.

What happens if we go anything above CR 25 : 1.

I am looking for more exploration

We can go above 25:1 in principle. But the cylinder pressures will rise exponentially (pressure at end of compression = p1 x CR^gamma) and the designers will have to increase the wall thickness accordingly, adding to the weight, bulk and cost. The temperatures will also go up, so metallurgical issues can crop up, apart from putting stress on the cooling and lubrication systems. Also, as mentioned earlier, the improvement in thermal efficiency and power output will be only marginal (recall that the slope of the efficiency vs. CR graph keeps reducing). So the power : weight ratio will reduce beyond a limit. There could be emission issues too (e.g. CO formation by CO2 dissociation).