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HARARE POLYTECHNIC
DIVISION OF AUTOMOTIVE
ENGINEERING

ENGINES
INLET MANIFOLDS

WRITTEN BY:- G.T.T. MAPENZAUSWA a.k.a. MUTOTI.

HARARE POLYTECHNIC – DIVISION OF AUTOMOTIVE ENGINEERING.
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WRITTEN BY:- G.T.T. MAPENZAUSWA a.k.a. MUTOTI

HARARE POLYTECHNIC.
DIVISION OF AUTOMOTIVE ENGINEERING.
ENGINES.

INLET MANIFOLDS.

OBJECTIVES

The student will after studying this module, be able to correctly in writing and without references:-
1. Describe the air / charge flow in inlet manifolds.
2. State:-
i. The function of the intake manifold in multi – cylinder engines.
ii. The effect of reverse flow on volumetric efficiency.
iii. Two ways of preventing reverse flow over a wide range of speed.
iv. Two ways of manifold design that improve V.E.
v. Effect of high R.P.M. on V.E.
vi. Why the fuel sprayed should be kept in suspension and effect of not keeping it in suspension.
3. List:-
i. Five factors influencing the suspension of fuel and the effect of each factor.
ii. Six features to be considered when designing inlet manifolds.
iii. Two disadvantages of long inlet manifolds.
4. Define ‘induction robbery’.
5. List:-
i. One advantage of the two – branch manifold with siamese inlet valve ports and a single
carburetor.
ii. Four disadvantages of the two – branch manifold with siamese inlet valve ports and a single
carburetor.
iii. Four advantages of the two – branch manifold with siamese inlet valve ports, twin carburetors
and balance pipe.
iv. Three disadvantages of the two – branch manifold with siamese inlet valve ports, twin
carburetors and balance pipe.
6. State:-
a.
i. Function of the balance pipe.
ii. Four advantages of the four branch semi – streamlined manifold with single carburetor.
WRITTEN BY:- G.T.T. MAPENZAUSWA a.k.a. MUTOTI.
HARARE POLYTECHNIC – DIVISION OF AUTOMOTIVE ENGINEERING.
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b. State:-
i. Three advantages of the two pairs of ‘V’ fork – type manifold with twin carburetors.
ii. One disadvantage of the two pairs of ‘V’ fork – type manifold with twin carburetors.
c. State:-
i. Four advantages of the four – branch streamlined manifold with a single carburetor.
ii. The function of the resonating chamber between the carburetor and the individual tracts.
d. State:-
i. Four advantages of the four – branch induction manifold with dual barrel carburetor.
ii. Four advantages of the multi – point fuel injection four branch manifold with plenum
chamber.
iii. Function of the plenum chamber.
7. State:-
i. Three disadvantages of the three branch manifold with siamesed inlet ports and single
carburetor.
ii. Function of the deflector.
iii. How induction robbery is avoided.
8. State:-
i. Three advantages of the six branch twin manifold with balance pipe.
ii. State how induction robbery is avoided.
9.
a. Define ‘tuning of inlet’ manifold.
b. State when is the inlet manifold tuned to be in phase.
10. State:-
i. The reason for inlet manifold heating.
ii. Three disadvantages of hot spotting.
11. Describe:-
a.
i. Well – type hot spotting.
ii. Thermostatic controlled hot spotting.
iii. Water – heating manifold.
b. State three disadvantages of using water for heating manifolds.

WRITTEN BY:- G.T.T. MAPENZAUSWA a.k.a. MUTOTI.

HARARE POLYTECHNIC – DIVISION OF AUTOMOTIVE ENGINEERING.
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INLET MANIFOLDS.
Function:-
i. To distribute the air from plenum chamber to the individual cylinders of a multi – cylinder engine.
ii. To distribute air / fuel mixture from the carburetor to the individual cylinders of a multi – cylinder
engine.
Air flow in the manifold:-
i. Towards the end of the induction stroke, the momentum of the fresh charge / air in the inlet pipe
causes the charge / air to flow towards the cylinder even after the piston has passed B.D.C.
ii. This inward flow of the charge / air, combined with the upward movement of the piston for the next
stroke causes a rise in pressure of the charge in the cylinder, which stops the inward flow of the
charge / air. This is the point at which the inlet valve should close.
iii. The pressure at the port end of the inlet pipe will be higher than the pressure at the open end and this
will cause the air / charge to REVERSE FLOW away from the closed valve.
iv. The reverse flow of the charge / air will continue until the pressure at the port end of the pipe falls
below atmospheric pressure and another reverse flow occurs towards the port end of the pipe. This
continues to happen as long as there is no opposition to flow, and the time for one complete
oscillation depends on the length of the pipe. Therefore the inlet valve should open when the flow is
towards the port end for better volumetric efficiency.
Remedy to reverse flow over a wide range of speed.
i. Use of short inlet manifold.
ii. Length of pipes between carb / plenum chamber and each cylinder should be the same.
Volumetric efficiency:-
For improved V.E.
i. The pipes should be of large diameter
ii. All bends within the pipes should be gentle.
Fuel spray:-
The fuel sprayed has to be kept in suspension in the air otherwise it would settle out on the floor and walls of
the manifold and never reach the cylinder. If from the floor it flowed into the cylinders, no combustion
would take place because liquid fuel does not burn.
Factors influencing suspension of fuel:-
i. Speed of flow → high speed, good suspension.
ii. Cross – sectional area of the manifold → small area, rapid flow.
iii. Degree of atomization of fuel → fineness.
iv. The pressure / depression in the manifold.
v. The temperature of the mixture in the manifold.
WRITTEN BY:- G.T.T. MAPENZAUSWA a.k.a. MUTOTI.
HARARE POLYTECHNIC – DIVISION OF AUTOMOTIVE ENGINEERING.
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Manifold designs:-
Manifolds are designed to provide the best possible distribution of mixture, both as to quantity and equality
of air / fuel ratio, among the individual cylinders.
Eg. Long in – line carburetor engines. [above 4cyl.]
- The distribution of fuel is such that the heavy particles of air / fuel mixture have greater inertia than
the lighter particles.
- As a result, the heavy particles will tend to continue moving past middle branches of the manifold.
- Therefore, middle cylinders receive lighter particles [run lean] and have tendency to detonate because
they will have received the least amount of tetraethyl lead.
- The end cylinders on the engine run rich because of the heavy fuel particles.
Disadvantages of long manifolds:
i. Poor fuel economy.
ii. Excessive exhaust emissions.
Remedy to above:-
i. Not too large diameter pipes. → to maintain reasonably high mixture speeds at low engine speeds.
ii. Where change in direction of flow takes place, sharp corners be used instead of gentle bends. →
turbulence created at corners assist in keeping the fuel in suspension but sharp corners reduce V.E.
iii. Use of short manifolds. → reasonable fuel economy, improved emissions and uniform power output
at all speeds.
iv. Means of heating manifolds → encourage evaporation.
v. Use fuel injection with plenum chamber.
vi. Use of fairly smooth walls but not polished.

INLET MANIFOLD LAYOUTS.

FOUR CYLINDER IN – LINE INLET MANIFOLD LAYOUTS.

Induction robbery:-
Is the starvation of the charge to one of the cylinders in a siamesed inlet port layout.

Two – branch manifold with siamese inlet valve ports and single carburetor. FIG.1a.

FIG.1a
Advantages:-
Simplified cylinder head casting.
WRITTEN BY:- G.T.T. MAPENZAUSWA a.k.a. MUTOTI.
HARARE POLYTECHNIC – DIVISION OF AUTOMOTIVE ENGINEERING.
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Disadvantages:-
i. Poor volumetric efficiency.
ii. Severe induction robbery.
iii. Poor distribution of mixture.
iv. High fuel consumption.
Two – branch manifold with siamese inlet valve ports, twin carburetors and balance pipe. FIG.1b.

FIG.1b.
Advantages:-
i. Good volumetric efficiency especially at high speeds.
ii. Fair distribution of mixture.
iii. Smooth idling because of equalization of depression by the balance pipe.

Disadvantages:-
i. Severe induction robbery.
ii. Not economic.[ because of the irregular air flow through the carburetor, requires slightly rich
mixtures]

Four – branch semi – streamlined manifold with single carburetor. FIG.1c.

FIG.1c.
i. The two outer branch elbows are curved to minimize the flow resistance but the two inner branches
merge with the main gallery to form a sharp edge tee junctions.
ii. Thus the longer flow path for the outer cylinders is partially compensated by the streamlining of these
branches so that the quantity of charge flowing into each inlet port is approximately equal.

WRITTEN BY:- G.T.T. MAPENZAUSWA a.k.a. MUTOTI.

HARARE POLYTECHNIC – DIVISION OF AUTOMOTIVE ENGINEERING.
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Advantages:-
i. No induction robbery.
ii. Very economic.
iii. Improved distribution of mixture.
iv. Improved V.E.

Two pairs of ‘V’ fork – type manifolds with twin carburetors. FIG.1d.

FIG.1d.
Advantages:-
i. Very good V.E. [high power output at high speeds]
ii. No induction robbery.
iii. Very good mixture distribution.

Disadvantage:-
High fuel consumption

Four – branch streamlined manifold with single carburetor. FIG.1e.

FIG.1e.
Resonating chamber between the carburetor and the individual tracts, amplifies the pressure pulses in the
manifold.

Advantages:-
i. Good V.E.
ii. No induction robbery.
WRITTEN BY:- G.T.T. MAPENZAUSWA a.k.a. MUTOTI.
HARARE POLYTECHNIC – DIVISION OF AUTOMOTIVE ENGINEERING.
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iii. Good mixture distribution.

iv. Very economic on fuel.

Multi – point fuel injection four branch – manifold with plenum chamber. FIG.1f.
The plenum chamber acts as an air reservoir and dampens the pressure pulsations.

Advantages:-
i. Very good V.E.
ii. No induction robbery.
iii. Very good fuel or air distribution [indirect injection:- fuel injected in individual pipes of cylinders at
the port entrance. In direct injection only air is distributed.]
iv. Very economic on fuel.

FIG.1f.

Four – branch induction manifold with dual barrel carburetor. FIG.1g.

FIG.1g.
Advantages:-
i. Good V.E.
WRITTEN BY:- G.T.T. MAPENZAUSWA a.k.a. MUTOTI.
HARARE POLYTECHNIC – DIVISION OF AUTOMOTIVE ENGINEERING.
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ii. No induction robbery.

iii. Very good mixture distribution.
iv. Very economic on fuel.

SIX CYLINDER IN – LINE ENGINES INLET MANIFOLDS.

Three branch manifold with siamesed inlet ports and a single carburetor. FIG.2a.

Advantage:-
Improvement on induction robbery → because of fitting the deflector.

Disadvantages:-
i. Poor V.E.
ii. Mixture distribution is very poor.
iii. Very high fuel consumption.

Six branch twin manifold with balance pipe and twin carburetors.
Advantages:-
i. Good V.E.
ii. Good mixture distribution with fitting of a deflector and carburetor for each three branch.
iii. Very low fuel consumption.
iv. No induction robbery because of the firing order → no firing of adjacent cylinders one after the other.
Disadvantages:-
Expensive design / arrangement.

WRITTEN BY:- G.T.T. MAPENZAUSWA a.k.a. MUTOTI.

HARARE POLYTECHNIC – DIVISION OF AUTOMOTIVE ENGINEERING.