href="#ulink_24f48dea-fe73-5ef2-a1dd-f2452451e540">Table 1.1 illustrates, the mean piston speed is about 12 m/s, the bmep is about 15 bar, the power/volume is about 40 kW/L, and the power/mass about 0.5 kW/kg for the three engines.
Table 1.1 Performance Comparison of Three Different Four‐Stroke Turbocharged Diesel Engines
| 1.9 L | 5.9 L | 7.2 L | |
| Parameter | Automobile | Truck | Military |
| # Cylinders | 4 | 6 | 6 |
| Bore (mm) | 82 | 102 | 110 |
| Stroke (mm) | 90 | 120 | 127 |
| Displacement per cylinder (L) |
|
0.983 | 1.20 |
| Power (kW) | 110 | 242 | 222 |
| Mass (kg) | 200 | 522 | 647 |
| Engine speed (rpm) | 4000 | 3200 | 2400 |
| Mean piston speed (m/s) | 12.05 | 12.78 | 10.16 |
| Bmep (bar) | 17.3 | 15.4 | 15.4 |
| Power/Volume (kW/L) | 57.9 | 41.0 | 30.8 |
| Mass/Volume (kg/L) | 105 | 88 | 90 |
| Power/Mass (kW/kg) | 0.55 | 0.46 | 0.35 |
There is good reason for this; all engines in a given era tend to be made from similar materials. The small differences noted could be attributed to different service criteria for which the engine was designed. Advances in engine technology have allowed manufacturers to continue to increase the power/mass. The iron in engine blocks and cylinder heads has been replaced by aluminum, which has half the weight of iron, and intake manifolds are now made of composite materials. With turbocharging, engines for vehicles have also become smaller, with four‐ and six‐cylinder engines replacing six‐ and eight‐cylinder engines, respectively.
Since material stresses in an engine depend to a first order only on the bmep and mean piston speed, it follows that for the same stress limit imposed by the material, all engines should have the same bmep and mean piston speed. Finally, since the engines geometrically resemble one another independent of size, the mass per unit displacement volume is more or less independent of engine size.
1.5 Engine Configurations
Internal combustion engines can be built in many different configurations. For a given engine, using a four‐ or two‐stroke Otto or Diesel cycle, the configurations are characterized by the piston‐cylinder geometry, the inlet and exhaust valve geometry, the use of super or turbochargers, the type of fuel delivery system, and the type of cooling system. The reciprocating piston‐cylinder combination remains the dominant configuration of the internal combustion engine.
Since the invention of the internal combustion engine, many different piston‐cylinder geometries have been designed, as shown in Figure 1.13. The choice of a given arrangement depends on a number of factors and constraints, such as engine balancing and available volume. The in‐line engine is the most prevalent because it is the simplest to manufacture and maintain. The V engine is formed from two in‐line banks of cylinders set at an angle to each other, forming the letter V. A horizontally opposed or flat engine is a V engine with 180
Figure 1.13 Various piston‐cylinder geometries. (Adapted from Obert 1950.)
Intake and Exhaust Valve Arrangement
Gases are admitted and expelled from the cylinders by valves that open and close at the proper times, or by ports that are uncovered or covered by the piston. There are many design variations for the intake and exhaust valve type and location. Poppet valves (see Figure 1.14) are the primary valve type used in internal combustion engines since they have excellent sealing characteristics. As shown in the pushrod configuration of Figure 1.14, springs are used to return the valve to a closed position. Sleeve and rotary valves have also been used, but do not seal the combustion chamber as well as poppet valves.
The poppet valves can be located either in the engine block or in the cylinder head, depending on airflow, cooling, and manufacturing considerations. Older engines and small four‐stroke engines have the inlet and exhaust valves located in the block parallel to the cylinders, a configuration termed under‐head or L‐head. This configuration provides good cooling to the valves from the engine block coolant, however with undersquare (bore
Currently, most engines use valves located in the cylinder head, an overhead or I‐head configuration, as this configuration has allows increased valve diameter resulting in good inlet and exhaust flow characteristics.
