During the overlap period where both inlet and exhaust valve are open is useful in clearing the combustion chamber from combustion products of the previous cycle. This period is variable in length. Does it have a negative or positive effect on the produced power?
Sir,
Yes definitely the overlap period will have considerable effect on the power output. If the overlap period is too high it results in loss of fuel through exhaust port without participating in combustion results in poor thermal efficiency and higher BSFC. Also after burning (in exhaust pipe ) can be seen in longer overlap periods. it increases the NOx to some extent.
On the other hand if the overlap period is too small results in improper scavenging and higher residual gas trapping inside the combustion chamber leading to higher CO emissions. To some extent it helps the auto ignition process in case of Ci engines.
You can find the papers dealing with "Negative valve overlap" technique for HCCI engines. also the effect of Early inlet valve opening (EIVO) and (early inlet valve closing (EIVC) similarly EOVC and EOVO effects. There are many articles dealing with these effect on IC engines.
I hope this answer satisfies you.
But Dr Sharma in the case of CI engines there will be no loss of fuel because as you know you will be admitting air only through the inlet valve. I wonder what will be the optimum length for each type of engine.
Yes Sir,
I this happens mostly in SI engines. But in case of CI engines early injection results in loss of power output, if the combustion occurs well before TDC.
A reasonable positive vale overlap helps to clean combustion residuals in engine cylinder, too much valve overlap would help to expel the fresh charges into exhaust pipe thus increase HC and CO emission and hence results in bad fuel economy. Too less or negative valve overlap help to retain the residual of the previous combustion cycle, such technique is so called "internal EGR". It is hard to tell which kind of valve overlap is better for engine running, it depends on the engine types and the aim of the users.
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