# Thread: volumetric efficiency of 4 cylinder 4 stroke engine calculation advice

1. I have been asked to calculate the following problem and I am struggling with calculating the volumetric efficiency part c. So far I have calculated:

A 4 cylinder 4 stroke engine of 78mm bore and 105mm stroke develops an indicated power of 47.5kW at 4400rpm. The air/fuel ratio is 21kg air/kg fuel, the fuel consumption is 13.6 kg/h and the calorific value is 41.8Mj/kg. Calculate:

a. mean effective pressure:

Using IP= 100 rho LAN (For kW)

gives : rho = IP/100LAN

= 47.5/ 100 * (PI*{0.078^2/4}* 0.105 * 73.3/2 = 47.5/1.838 = 25.84 bar

b. Indicated thermal efficiency

Using eta Ith = IP/FP where FP = Mf x cv

eta Ith= IP/mf * cv

eta Ith= 47.5 * 10^3/ 0.037*41800 = 0.3071 = 30.7%

c. Volumetric efficiency given inlet conditions 1 bar and 15 deg C. R=287 J/kgK.

I have found out that volumetric efficiency is the ratio of actual air mass drawn into the engine during one cycle to that of atmospheric conditions.

eta volumetric = actual mass of air/ rho(atmosheric conditions) * displacement

or

eta volumetric = rev per cycle * steady air flow in / rho(atmosheric conditions) * displacement * engine speed

I am struggling to find the actual mass of air drawn in or steady air flow in any help will be greatly appreciated.

Thanks James

2. Can you post the problem?

3. Hi Jilan the problem is:

A 4 cylinder 4 stroke engine of 78mm bore and 105mm stroke develops an indicated power of 47.5kW at 4400rpm. The air/fuel ratio is 21kg air/kg fuel, the fuel consumption is 13.6 kg/h and the calorific value is 41.8Mj/kg. Calculate the Volumetric efficiency given inlet conditions of 1 bar and 15 deg C. R=287 J/kgK.

4. Good, do you have any equations that might help us?

5. I have found out that volumetric efficiency is the ratio of actual air mass drawn into the engine during one cycle to that of atmospheric conditions.

eta volumetric = actual mass of air/ rho(atmosheric conditions) * displacement

or

eta volumetric = rev per cycle * steady air flow in / rho(atmosheric conditions) * displacement * engine speed

6. So we are comparing the amount of air used with the amount of air drawn into the engine. We are doing to need to convert the volume of air to a mass. Can you calculate the density of air so we can do that?

7. Hi Jilan, It's just clicked how to do it using the ideal gas equation I can calculate the density then convert all units to si form then plug the figures in to the above equation! ! Can't believe I have overlooked that thanks for the light bulb moment I needed.

Regards James

8. PV=nRT rears its pretty head again.

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