Page 106 - 05. Subyek Teknik Mesin - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

2
90 Automobile mechanical and electrical systems

wheel torque. The mounting system therefore includes mountings to support
weight and counteract torque separately. These are mounted at the top or
bottom of the engine, respectively.

2.1.2 Engine operating details

The air above the Earth’s surface is like a fl uid that exerts a pressure on all points
around it because of the Earth’s gravitational force pulling it down. This creates a
pressure known as atmospheric pressure and is 101.325 kPa, 760 mmHg, 29.92
inches Hg, 14.696 psi or 1013.25 millibars (let’s stick with approximately 1 bar or
15 psi).
A naturally aspirated engine (one that does not use forced induction by a
Defi nition supercharger or turbocharger) relies on atmospheric pressure to charge the
cylinder with gas (air or air/fuel mixture) ready for the combustion process. As
Naturally aspirated engine
the piston moves down the cylinder (from TDC to BDC), the volume increases
An engine that does not use forced
induction by a supercharger or and this causes the pressure in the cylinder to reduce, becoming lower than
turbocharger. atmospheric pressure. This creates a pressure difference between the inside
and outside of the cylinder, and as a result the atmospheric pressure (the higher
pressure) forces gases into the cylinder (where there is lower pressure) until the
pressure is balanced. Note that any restriction to the fl ow of gas will reduce the
effectiveness of the cylinder charging process.
Volumetric effi ciency is a measure of the effi ciency of the cylinder charging
Defi nition
process during the induction stroke. Theoretically, the cylinder should be
completely fi lled with a mass of gas, but in practice this never happens owing to
Volumetric effi ciency
fl ow losses and ineffi ciencies. Therefore, the volumetric effi ciency is a measure
A measure of the effi ciency of the
cylinder charging process during the of the actual amount of gas induced compared to the theoretical amount (which
induction stroke. is the mass required to completely fi ll the cylinder volume) and is expressed as a
percentage ( Fig. 2.17 ). It is calculated as:
(Actual mass of air/Theoretical mass of air 100 %
)

actual mass of air
volumetric efficiency = X 100
possible mass of air

Top dead centre (tdc)

Bottom dead centre (bdc)

Cylinder filled with air at Cylinder filled with air below
atmospheric pressure atmospheric pressure and less dense

Figure 2.17 Volumetric effi ciency