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In this section, we will look into three engine efficiency topics: Volumetric
Efficiency, Thermal Efficiency and Mechanical Efficiency.
Volumetric Efficiency
|
Actual
CFM |
= Volumetric Efficiency |
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Theoretical CFM |
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Volumetic Efficiency or VE, I will be using from this point, varies depending
on temperature and pressure.
From that, we know a normally aspirited engine will have VE of 100% or
less. And force inductioned engine will have VE of 100% or more.
The actual calculation of VE is done by ECU using measured amount of intake
air, with Mass Air Sensor measuring at intake pipe or Speed Density measuring inside the intake manifold (close to intake
port of the engine).
theoretical cfm = rpm x displacement / 3456
Engine Flow= (engine displacement) X (volumetric efficiency) X (engine speed)
X (manifold pressure)
You can see the key to increase engine flow is to increase engine VE
(volumetric efficiency). Reduce intake charge temperature is the easiest way to help increase engine VE. This is where air/air,
air/fluid intercooler and water injection come into play.
Assuming VE at 100%, 1 atmosphere pressure, we have the following table
for a 3.0L engine flow( CFM) at various rpm and pressure point.
- Influence of the altitude above Sea Level on the
Volumetric Efficiency
|
Influence
of the elevation above Sea Level on the Volumetric Efficiency
Atmospheric
pressure as given by average barometer reading |
| Height
above sea level, ft. |
Atmospheric
pressure, in. of mercury |
Atmospheric
Pressure, in PSI (approximate) |
Relative
volumetric efficiency |
| 0 |
29.92 |
14.7 |
1.000 |
| 1,000 |
28.85 |
14.2 |
0.965 |
| 2,000 |
27.82 |
13.7 |
0.931 |
| 3,000 |
26.82 |
13.2 |
0.892 |
| 4,000 |
25.85 |
12.7 |
0.865 |
| 5,000 |
24.92 |
12.2 |
0.833 |
| 6,000 |
24.00 |
11.7 |
0.803 |
| 8,000 |
22.17 |
10.7 |
0.742 |
| 10,000 |
20.34 |
8.7 |
0.681 |
| 12,000 |
19.30 |
6.7 |
0.645 |
Most engine control systems in production today utilize either
speed-density sensors or air-mass sensors to measure engine air flow. Speed-density is very popular because of its low cost
and high reliability. Speed-density systems typically calculate engine flow rate based on engine speed, intake manifold pressure
and temperature. Some systems use barometric pressure sensors and inlet air temperature sensors to improve flow calculation
accuracy for varying ambient conditions. Fig. 1 illustrates a typical engine with both speed-density sensors and an air-mass
sensor.
Figure 1: TBI Engine with Air-Mass Sensor and Speed-Density
Sensors*
eed-density systems calculate an air flow rate that approximates
the flow rate at At the intake ports, mdot_ao. Air-mass sensor systems usually measure the air flow rate near the throttle
and thus approximate mdot_ai. During throttle transients (or whenever the manifold pressure fluctuates) the flow rates
in and out of the manifold are different from one another, i.e.,  . This is illustrated in Fig. 2 for a rapid throttle transient.
Figure 2: Air flow rates during a rapid throttle transient
The large in-rush of air during the throttle opening is often
referred to as "manifold filling." During this time the flow rate into large manifolds can be several hundred percent higher
than the flow rate at the ports.
Speed-density systems calculate air density in the intake
using manifold pressure and temperature sensors and the perfect gas law. The air-mass flow rate at the intake ports, mdot_ao,
is assumed to be quasi-steady and may be calculated as:
The volumetric efficiency, , of the engine is usually mapped from steady-state air flow measurements and stored in the controller's Read Only Memory
(ROM) as a table having inputs of manifold pressure and engine speed as shown in Fig. 3:
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