Piston Engine

Question 1

Explain what is meant by mean effective pressure

Answer 1

It is a fictitious pressure that, if it acted on the piston during the entire power stroke, would produce the same amount of net work as that produced during the actual cycle.

Question 2

Describe the purpose and operation of inlet and exhaust valves in the piston engine. How are these valves driven?

Answer 2

The air-fuel mixture is drawn into the cylinder through the intake valve, and the combustion products are expelled from the cylinder through the exhaust valve

The opening and closing of the valves is controlled by cams which are fixed to a camshaft or shafts. The camshaft is operated by means of a gear, belt or chain drive from the crankshaft.

Question 3

Describe why the indicated power based on the Otto cycle may not be accurate in thrust based calculations.

Answer 3

• Combustion/exhaust processes are not instantaneous (rounds off the corners of the cycle)
• Valves take time to open and close (rounds off the corners of the cycle)
• Heat losses to sidewalls, valves, etc
• Finite pumping work (shown as a negative work loop on the p-V diagram.
• Incomplete displacement of exhaust gas with fresh charge due to engine capacity and ambient pressure conditions

Question 4

What is meant by an overhead camshaft engine? Why is the camshaft geared down 1:2 to the crank-shaft in a four stroke engine?

Answer 4

An Overhead cam-shaft engine is the same as an Overhead valve engine, however, the camshaft is set up above the valves.

In order to complete the cycle, the piston has made four strokes, two up and two down; hence its name the four-stroke cycle.

At the same time the crankshaft has made two revolutions. For this cycle then, there is one power stroke in two revolutions. The valves open once only during the cycle, so the cams must rotate once only in two revolutions of the crankshaft. Thus the camshaft is geared down 1:2 to the crankshaft

Question 5

Describe the operation of a two stroke piston engine

Answer 5

As the name implies, all the events in the two-stroke cycle are
completed in two strokes. In two strokes the crankshaft makes one revolution, so the two stoke cycle is completed in one revolution

Question 6

Describe the operation of a 4 stroke piston engine

Answer 6

Four stroke engines complete all the events in a four-stroke cycle, two up and two down in two revolution.

• intake stage: the inlet valve is fully open and the exhaust valve closed. The piston is descending so it is sucking a fresh air-fuel charge into the cylinder through the open inlet valve. Toward the bottom of the suction stroke the inlet valve begins to close.
• Combustion stage: both the inlet and exhaust valves are closed. The piston is ascending and is compressing the fresh air-fuel charge into the combustion space. Ignition of the charge occurs towards the top of this stroke; once the stroke is completed, the piston again begins to descend on the power stroke.
• Power stroke stage: both the inlet and exhaust valves are again closed. The air-fuel charge has been ignited and the combustion products are rapidly expanding, pushing the piston down on its power stroke. Toward the bottom of the power stroke the exhaust cam has rotated such that it begins to lift the exhaust valve while the inlet valve remains closed.
• Exhaust stage: the exhaust valve is fully open and the piston is ascending. As the piston ascends, it pushes the combustion products out through the open exhaust port. Towards the top of the exhaust stroke the exhaust valve begins to close and the inlet valve begins to open as a result of cam action.

Question 7

What methods have traditionally been applied to increase piston engine output, and what are the associated problems?

Answer 7

• Piston engine output can be increased by supercharging.
• Evaporative cooling: Evaporative cooling of the intake charge by excess fuel and/or water injection raises the density, the charge-mass flow, and hence the power output.
• Increasing engine speed: Other things being unchanged, power output is directly proportional to engine speed n. However, the penalty is increased piston speed and therefore engine wear.
• Increasing engine compression ratio: Power output rises in direct proportion to bmep. However, again, increased mechanical loading of the engine may degrade service life. Also, fuel octane rating of Otto cycle engines will place an upper limit on usable compression ratio r.
• Improving volumetric efficiency: This can be obtained with multiple valves and manifolds. Such designs “breathe easier”, increase volumetric efficiency, and increase the charge-mass flow.
• Increasing the number of cylinders: If the physical aspects of the cylinder are optimised, power output can be increased only by adding more cylinders. The number has gone as high as 28. however, it becomes more mechanically complex and reduces its reliability. Further, significant improvement in specific power output has only been achieved by supercharging.

Question 8

Derive the charge-mass law for piston engines, stating clearly any assumptions made.

Answer 8

[Assumptions]
- Reff = effective gas constant of both the charge and exhaust gases is equal to 0.99 of that of Air
- the heat received by the fresh charge is that supplied by the residual exhaust

Check Page 115 on the PDF

Question 9

Derive an equation for thrust based on the Rankin-Froude actuator disk model.

Answer 9

https://app.box.com/s/zrhyh546jdezkzpn8ydytnyht5tunn6t/file/476131739148

Q4 Part D

Question 10

Draw pressure-volume diagrams for the two stroke and four stroke piston engines indicating the main features of the cycle.

Answer 10

Q4 part B

https://app.box.com/s/zrhyh546jdezkzpn8ydytnyht5tunn6t/file/476131461101

• The two-stroke engine diagram shows the reduction in pressure in the expansion line as the exhaust port opens. Also, the similarity of shape between the indicator diagram and the theoretical constant volume cycle is much more apparent than the four-stroke engines.
• In the four-stroke engine diagram, compression, ignition and expansion appear in a similar manner to the two-stroke cycle. But after expansion in this cycle, there is a definite exhaust stroke which will appear as a line on the P-V diagram. The atmospheric pressure line is slightly below the exhaust pressure line.

Question 11

Describe briefly the evolution of the piston engine in the first half of the 20th century, including the reasons why larger engines were gradually replaced by gas turbine engines.

Answer 11

By the 1950s, the power/weight ratio and power output of piston engines significantly increased due to the higher speed and performance demand for military aircraft; and the need for increased speed, reliability, longer life and greater safety in the operation of civil transport aircraft.

The specific weight of civil transport engines had been reduced from 9kg/kW of the original Wright brother’s engine to 0.6kg/kW and the power of operational engines had peaked at 4300hp (3200kW). It was during this period however that piston engines began being supplanted by gas turbines.