Engine Desgin and Performance Data

2.15 Engine Desgin and Performance Data

Engine ratings usually indicate the highest power at which manufacturers expect their products to give satisfactory economy, reliability, and durability under service conditions. Maximum torque, and the speed at which it is achieved is Usually given also.

Since both of these quantities depend on displaced
volume, for comparative analyses between engines
of different displacements in a given engine category
normalized performance parameters are more useful.

The followmg measures, at the operating points
indicated, have most significance :

Typical performance data for spark-ignition and diesel
engines over the normal production size range are
summarized in following table .

1. At maximum or normal rated point:

Mean piston speed Measures comparative success in handling loads due to inertia of the parts, resistance to air flow, and/or engine friction.

Brake mean effective pressure , In naturally aspirated engines bmep is not stress limited. It then reflects the product of volumetric efficiency ( ability to induct air ) . fuel/air ratio (effectiveness of air utilization in combustion ) , and fuel conversion efficiency.

In supercharged engines bmep indicates the degree of success in handling higher gas pressures and thermal loading .

Power per unit piston area , Measures the effectiveness with which the piston area is used, regardless of cylinder size .

Specific weight , Indicates relative economy with which materials are used.

Specific volume , Indicates relative effectiveness with which engine space has been utilized.

2. At all speeds at which the engine will be used with full throttle or with maximum fuel-pump setting:

Brake mean effective pressure Measures ability to obtain/provide high air flow and use it effectively over the full range.

3. At all useful regimes of operation and particularly in those regimes where the engine is run for long
periods of time:

- Brake specific fuel consumption or fuel conversion efficiency.
- Brake specific emissions

Typical performance data for spark-ignition and diesel engines over the normal production size range are summarized in Table above .

The four-stroke cycle dominates except in the smallest and largest engine sizes. The larger engines are turbocharged or supercharged. The maximum rated engine speed decreases as engine size increases, maintaining the maximum mean piston speed in the range of about 8 to 15 m/s.

The maximum brake mean effective pressure for turbocharged and supercharged engines is higher than for naturally aspirated engines.

Because the maximum fuel/air ratio for spark-ignition engines is higher than for diesels, their natutally aspirated maximum bmep levels are higher. As engine size increases, brake specific fuel consumption decreases and fuel conversion efficiency increases, due to reduced importance of heat losses and friction.

For the largest diesel engines, brake fuel conversion efficiencies of about 50 percent and indicated fuel conversion efficiencies of over 55 percent can be obtained.