《发动机原理》双语教案

Chapter 2 Engine Gas Exchange Processes

Key points: Gas exchange process of four-stroke internal combustion engine and ventilation losses, the concept of combustion engine filling coefficient.

Difficult points: The measures to improve internal combustion engine filling quantity coefficient.

2.1 Inlet and exhaust process of the four-stroke engine

The purpose of the exhaust and inlet processes or of the scavenging process is to remove the burned gases at the end of the power stroke and admit the fresh charge for the next cycle.

Inducting the maximum air mass at wide-open throttle or full load and retaining that mass within the cylinder is the primary goal of the gas exchange processes.

Engine gas exchange processes are characterized by overall parameters such as volumetric efficiency for four-stroke cycles.

2.1.1 Gas exchange process of four-stroke engine

The gas exchange process of an engine consists of the duration from opening the exhaust valve to closing the inlet valve, it extends approximately 410~480oCA and can be divided into four phases—blowdown, displacement, induction and scavenging.

1.Blowdown

The duration from exhaust valve opening to cylinder pressure closes to pipe pressure is referred to as blowdown phase.

The burned cylinder gases are discharged due to the pressure difference between the cylinder and the exhaust system. If the exhaust valve begins to open when the piston reached BDC, back pressure against the upward piston must be extremely high. Thus the exhaust process usually begins 40~60oCA before BDC (exhaust lead crank angle).

Blowdown phases ends when pressure difference between the cylinder and the exhaust system disappears, about 10~30oCA after BDC. Though the free exhaust phase covers only about 1/10 of exhaust stroke, it discharges 60% of burned gas.

2. Displacement

The exhaust gas is scavenged by piston’s upward motion that is the burnt gas is forced out of the cylinder. It is a positive displace process.

The exhaust valve closes 15~30oCA after TDC (exhaust lag crank angle) to improve emptying the cylinders and make the best use of the inertia of the gases in the exhaust systems.

3. Induction process

From the inlet valves open to close, the whole process that internal combustion engine inhales fresh charge is called intake process.

The usual practice is to extend the valve open phases beyond the intake strokes to improve charging of the cylinders and make the best use of the inertia of the gases in the intake systems.

The intake valve opens 10~20oCA before TDC (the inlet lead crank angle) and closes 40~70oCA after BDC (the inlet lag crank angle).

4. Valve overlap and scavenging

The exhaust valve closes 15 to 30oCA after TDC and the inlet valve opens 10 to 20oCA before TDC. The duration that both valves are open are called an overlap period.

With both valves opening, the inertia of fresh charge can be used to sweep the exhaust gases out of cylinder without any loss if the overlap is proper, that is so called scavenging.

The advantage of valve overlap occurs at high engine speeds when the longer valve-open periods improve volumetric efficiency. If the valve overlap is too large, backflow of exhausted gas into the cylinder gases into the intake will usually occur.

2.1.2 Valve timing

The valve timing are modified to set better charging and exhausting performance as there is always a difference between theory and practical.

2.2Volumetric efficiency

One of the most important processes that govern how much power and performance can be obtained from an engine is getting the maximum amount of air into the cylinder during each cycle.

1.Definition of volumetric efficiency

V olumetric efficiency is a measure of the effectiveness of the induction and exhaust processes.

In terms of quantities applying to an actual engine, volumetric efficiency is

defined as the mass of fresh mixture which passes into the cylinder in one suction stroke, divided by the mass of this mixture which would fill the piston displacement at inlet density.

s a v V V m m 1==η (2.1) Where m a = mass of air inhaled per cylinder per cycle;

m s = mass of air to occupy swept volume per cylinder at “ambient” pressure and temperature;

V 1= volume of “ambient ” air inhaled per cylinder per cycle;

V s = cylinder swept volume.

2. The influence factors of volumetric efficiency

When inlet valve closed, the overall volume of cylinder is V s ’+V c , the mass of trapped working fluid is m a : a s c a V V m ρ)'(+= (2.2)

Mass of residuals when exhaust valve closed: r r r V m ρ= (2.3)

From (2.1),(2.2) and (2.3) ,fresh charges inhaled per cylinder per cycle is: ()r r a s c s s v V V V V ρρρη-+=' (2.4)

Considering influence of intake and exhaust valve lag angle, make s c s c V V V V ++='ξ, c r V V =?,then: )()1(1r a s v ?ρξερρεη--= (2.5) By applying Ideal Gas Equation )/(RT p =ρ to Eq.(2.5): )(11r r a a s s v T p T p p T ?ξεεη--=

(2.6) The level of exhaust residuals trapped in the cylinder has a significant effect on the cycle-by-cycle variations in combustion, and the emissions of NO X . The residual coefficient γ is defined as mass of residual trapped in cylinder at the end of intake stroke, divided by mass of fresh charge inhaled in the intake stroke. It is used to

evaluate the residual percentage of mixture in cylinder. From (2.3) and (2.4):

a r r c a a r c r r a s c r r s s v r V V V V V V V V m ρερ?ξρρ?ρξρ?ρρρρηγ-=-=-+==)'( (2.7) By applying (2.6) to (2.7):

γεε

ξη+-=111a a s s v T p p T In a qualitative analysis, volumetric efficiency ηv increases with:

(1) Increasing mixture pressure at the end of intake stroke p a ;

(2) Decreasing mixture temperature at the end of intake stroke T a ;

(3) Reducing residual coefficient γ;

(4) Increasing compression ratio ?;

(5) Suitable valve parameters ξ and φ.

2.3 Effect of operating conditions and design on volumetric efficiency

1. Inlet Mach index:

For convenience the ratio of the typical velocity to the inlet sonic velocity, u/α, is called the inlet Mach index. The gas velocity is chosen by the following equation: i i p

p A C V A u =

Where u= gas velocity through the inlet valve at smallest area;

A p = piston area; V p = mean piston speed;

A i = nominal intake valve opening area; C i = inlet valve flow coefficient. And: α

ααi p i i p

i p C V D b C V A A u

Z 2)(=== Where Z= inlet valve Mach index; α= inlet sonic velocity;

b= cylinder diameter; D i = inlet valve diameter.

From a great number of experiments, it could be seen that the maximum volumetric efficiency is obtainable for an inlet Mach number of 0.55. Therefore, engine designers must take care of this factor to get the maximum volumetric efficiency for their engines.

2. Effect on intake system friction

During the intake stroke, due to friction in each part of the intake system, the pressure in the cylinder p c is less than the atmospheric pressure p atm by an amount dependent on the square of the speed.

j j c atm v k p p p p ρ∑=∑?=-=?

Where k= the resistance coefficient for that component which depends on its

geometric details; ρ= density of fresh air; v j= the local velocity.

3.Effect of inhaled charge heating

Designs which minimize the temperatures of inlet manifolds, inlet ports, inlet valves and valve seats are desirable. Improvement of heat conductivity between these parts and the coolant is effective in reducing temperature of fresh charge.

4.Effect of speed and valve timing

Flow effects on volumetric efficiency depend on the velocity of the fresh mixture in the intake manifold, port, and valve. Frictional flow losses increase as the square of engine speed, at higher engine speeds, the flow into the engine becomes choked.

Earlier-than-normal inlet valve closing reduces back-flow losses at low speed and increases ηv. Later-than-normal inlet valve closing, results in a decreasing inη at low engine speeds due to backflow.

5.Effect of intake runner length

The high volumetric efficiencies can be obtained at certain speeds by means of long inlet pipes. The effects noted are caused by the inertia and elasticity of the gases in the inlet pipe and cylinder.

As pipes become shorter, the maximum gains in volumetric efficiency grow smaller, but the range of speeds over which some gain is made grows wider.