effect of valve diameter

Diesel or petrol engine?

CE DesignerDiesel or petrol engine?

petrol engine

Decreasing valve dia upto a certain limit will increase engine performance of petrol engine . This is so because as valve dia dec. , the pressure of charge at inlet manifold will decrease due to increase in flow velocity. This will result in decrease in temperature of charge, reducing chances of preignition and knocking. So you can easily increase the compression ratio resulting in more power output in same fuel consumption... Thus bsfc increases. The efficiency also increases as a result .

Decreasing the dia too much will result in problems bcoz the charge temp will dec. too much making it difficult for fuel to reach its self ignition temperature.

S.mukherjeeDecreasing valve dia upto a certain limit will increase engine performance of petrol engine . This is so because as valve dia dec. , the pressure of charge at inlet manifold will decrease due to increase in flow velocity. This will result in decrease in temperature of charge, reducing chances of preignition and knocking. So you can easily increase the compression ratio resulting in more power output in same fuel consumption... Thus bsfc increases. The efficiency also increases as a result .

Decreasing the dia too much will result in problems bcoz the charge temp will dec. too much making it difficult for fuel to reach its self ignition temperature.

So, it probably means that whatever engines are manufactured their valve dia is already upto that 'certain limit' of getting maximum engine performance.
Correct me if I am wrong.

Less fuel = Less compression = less power

CE DesignerLess fuel = Less compression = less power

I seriously doubt that. Its because with decreasing valve dia the flow velocity will increase, provided the inlet passage dia remains constant. So amount of fuel shall remain the same.

I misinterpretted. I thought valve and passage were one and the same.

mechkyI seriously doubt that. Its because with decreasing valve dia the flow velocity will increase, provided the inlet passage dia remains constant. So amount of fuel shall remain the same.

If there is to be a change in the valve dia then the passage dia should be changed accordingly in order that the valve has a leak proof seat on it.

By reducing the inlet valve dia there should be a subsequent reduction in the passage dia. The flow velocity is increased during the suction stroke and turbulence is created in the combustion chamber which is essential for proper and complete combustion of the fuel-air mixture. If the exhaust valve dia is decreased then the resistance of the combustion products to flow out is increased as a result the stresses within the cylinder and especially on the piston is increased.