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I/C Engine Valve sizing study

Kevin Helton, Elijah Truitt, Logan Thorup

The objective of this project is to optimize the power output of a 4 stroke Otto cycle with a 2

valve flathead configuration by making modifications to the geometry of the valve layout. In this studythe effects of intake flow and exhaust flow will be considered. By changing the number of intake and

exhaust valves, or the size of existing valves, the engine power can be changed. While modifications will

be made to the base model, restrictions due to cylinder geometry must be considered. Keeping the size

of the bore diameter, stroke, and rod length the same, modifications may be made to the cylinder head

to increase surface area for the valve ports, although this may not be explored.

Conditions and processes for base case model:

Process 1 2 is an adiabatic, isentropic compression process Process 2 3 constant volume combustion process with q in = 2000 kJ/kg, to max T & P Process 3 4 is an isentropic, adiabatic process known as the power stroke Process 4 1 is a constant volume process with q out

The Cold air model was developed assuming inlet air at atmospheric pressure and a

temperature of 300K. The mass flowing into the cylinder is considered to be the same as the mass

exiting; consequently the mass in the cylinder is a function of the crank angle . The mass in the system

will also depend on the engine speed and inlet areas. This can be seen by comparing the mass in the

system at 4000 RPM and at 5000 RPM respectively. The mass is 0 .000534 kg at 4000 RPM and only

0.000496 kg at 5000 RPM.

Considering the base model at 4000 RPM, the inlet valve diameter is 28 mm and executes a maxlift of 7 mm. These conditions provide a maximum flow area of .000616 m. The intake process executes

through the first 180 of crankshaft rotation, therefore it makes sense that the mass will change for

different engine speeds, because the valve is open for different lengths of time. The exit valve diameter

is 24mm and has a maximum lift of 6mm. Under these conditions the maximum flow area for the

exhaust is 0 .000452 m. The exhaust process executes between 540 and 720 of crank rotation. Both

processes occur through a 180 duration, which means that both valves are open for the same amount

of time. Consequently the pumping power must increase for each positive change in area between the

intake and exhaust valve flow areas. Because of this, the flow from the exhaust reaches speeds much

higher than the speeds of the intake, this results in an overall net power decrease.

Modifications can be made to the base case to handle this problem an example would be to

make the intake and exhaust valves the same size. If a 2 valve configuration is to be used, the valve sizes

could be maximized to within the cylinder bore limits and the net power will increase. If the net power

increases and q in is held constant, then the thermal efficiency also increases. Also if the valves are of

equal size the pumping power decreases and if the net power of the cycle increases, the mechanical

efficiency increases as well.

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Alternative Case 1:

The first alternative case considered modifies the size and number of valves from the base

model. By decreasing the size of the original intake valve, there was enough space to add another valve.

With two intake valves with a diameter of 20 mm, the maximum flow area increased from 0.000616 m

to 0.00088 m. Though another valve was added, the decreased valve diameter allowed for the exhaust

valve to be increased from 24mm to 28 mm, resulting in a flow area of 0.000528 m. Taking the case of

4000 RPM, modifications made to the base model increased the net power out by approximately 7.5

KW. It is shown below from tabulations at different engine speeds that the maximum engine power for

the base model occurs at 5000 RPM and is 72.139 kW. In comparison, the maximum engine power for

the alternative case 1 occurs at 7000 RPM and is 91.017 kW.

Alternative Case 2:

The second alternative that we explored was based on the idea from above, that the pumpingpower will decrease when the flow area for the intake and exhaust are the same. It was decided to try

and achieve a 4 valve configuration in which there are two intake and two exhaust valves, all of equal

size. From cylinder geometry, it was found that the maximum valve size that could be achieved is 20

mm. The maximum lift of the intake and exhaust valves are slightly different, consequently so are the

flow areas. The flow area for the intake reaches a maximum of 0.00088 m and the exhaust reaches

0.000754 m. Again looking at the case of 4000 RPM, the modifications made to the base model

increased the net power out by approximately 9.75 KW. Again the maximum engine power for the base

model occurs at 5000 RPM and is 72.139 kW. In comparison, the maximum engine power for the

alternative case 2 occurs at 7000 RPM and is 97.186 kW.

From this study and for the constraints listed above, it has been shown that valve configuration

is very important to the cycle results. It has been shown that an increase in flow area, which ultimately

increases the mass in the cylinder, yields a greater engine power output. It has also been shown that by

configuring the intake and exhaust valve flow areas to be more close to equal, the power out and cycle

efficiency increase. Another possible option that could have been explored would be to change the

cylinder head shape. An example of this would be to make the head wedged or even dome shaped. This

would increase the surface area allowing for larger intake and exhaust ports. With larger flow areas, as

mentioned above the net power of the cycle can be increased.