IC Engine Terminology Learning Goal: To understand the relationships between real engine parameters and the corresponding model cycle and to be able to calculate the measures of performance for an engine. A four-stroke internal combustion engine operates with an unsteady open process. 1. Intake stroke (Figure 1) 2. Compression stroke (Figure 2) 3. Ignition (Figure 3) 4. Power stroke (Figure 4) 5. Blow down (Figure 5) 6. Exhaust stroke (Figure 6) These processes can be modeled by an idealized closed cycle with four states. This model gives a reasonable picture of the best possible performance of an engine. Ap-v diagram of this process can be seen below (Figure 7). 1. Isentropic compression (compression stroke) 2. Heat addition (ignition) 3. Isentropic expansion (powerstroke) 4. Heat rejection (blow down, exhaust, and intake) The cycle parameters must be related to the engine parameters. In an engine, the circular piston moves between two positions, top dead center (TDC) and bottom dead center (BDC). At TDC. the volume is at a minimum and is called the clearance volume Vole At BDC, the volume is at a maximum. The difference between these two volumes is the displacement volume Vold The distance the piston moves is called the stroke. and the diameter of the cylinder is called the bore. The cycle models one cylinder. The VOBDC compression ratio is VOLDC The total power produced for a four-stroke engine is Wet,total CWCWatere is the number of cylinders and is the rotational speed of the engine. The rotational speed of the engine needs to be divided by two since two rotations occur over the cycle (one rotation happens during the blow down, exhaust, and intake portion). Since the rotational speed of the engine is rotations per unit time, the dot over Weds incorporated in this quantity. Thus, W the work per cylinder per cycle, delivered every other revolution. This power can be related to the torque T by Wetto NT Part A Clearance volume An eight-cylinder engine has a bore of 100 mmand a stroke of 90 mm. If the compression ratio is 7.8, then what is the clearance volume? Express your answer with appropriate units to three significant figures. Hints Vol Value Units Submit My Answers Give Up It is often desirable to compare one engine's performance to another's, but because of the wide variation in parameters like the number of cylinders, the stroke, and the bore, direct comparisons are difficult. Several parameters have been developed to enable more useful comparison between engines with very different configurations. Whet The mean effective pressure is the ratio of the network to the displacement volume, MEP which has units of pressure. A larger MEP means that an engine produces more power per unit volume of displacement. Part B Mean effective pressure The engine from Part A is running at 2500 rpes and generating a torque of 750 N-m. What is the mean effective pressure? Express your answer with appropriate units to three significant figures. Hints 123- ? Value Units Submit My Answers Give Up Another measure that is useful is the specific fuel consumption, stc. This is the ratio of the fuel flow rate to the total net power, or afc= For a fuel that releases rom heat per unit mass, the thermal efficiency Wattal can be calculated as Figure of 7 Part C - Thermal efficiency Suppose the engine from Parts A and B is consuming fuel at 2x102 kg/sand the fuel has a heating value of 5 38. What is the thermal eficiency of the engine? Express your answer to three significant figures. ▸ Hints TDC BDC Vol Submit My Answers Give Up ? Provide Feedback Continue