The Function of The Intake Manifold of An Internal Combustion Engine

Its main function is: to introduce and seal the freshly mixed combustible mixture from the carburetor into the crankcase of the engine, and perform pre-compression.

Now, let's break down its specific functions and design features in detail:

Main function/Role

1. Channel Function (Basic Function):

· Connects the carburetor and the engine crankcase to provide a flow path for the fresh mixture (air + gasoline).

· In a four-stroke engine, the manifold connects the carburetor/throttle body and the intake port of the cylinder head; while in a two-stroke engine, it is directly connected to the crankcase.

2. Sealing and Containment Function (Key Function):

· In a two-stroke engine, the crankcase is used as a pump to pre-compress the fresh mixture. The intake manifold is the inlet of this "pump".

· It must provide absolute airtightness. If there is leakage, it will result in:

· Insufficient pre-compression pressure in the crankcase.

· Insufficient mixture (additional air enters), causing the engine to be weak at high speed, overheat, and even seize.

· Difficulty starting, unstable idle.

3. Providing Installation Platform:

· The carburetor is usually directly installed on the intake manifold, and it is also the installation seat for the diaphragm valve.

· For engines using piston valves for intake (such as many small-displacement motorcycles), the manifold is a simple channel connecting the carburetor and the crankcase housing.

4. Damping and Stabilizing Airflow:

· A certain manifold volume can slightly smooth out the pulsating airflow generated by the carburetor, making the mixture more uniform.

· Its internal shape affects the air flow speed and the fuel atomization effect.

The fundamental difference from the four-stroke intake manifold

Features: Two-stroke intake manifold, Four-stroke intake manifold

Connected objects: Carburetor → Crankcase (or diaphragm valve chamber) Throttle body/carburetor → Cylinder head intake port

Internal flow: Fresh oil-gas mixture Only air (for electronic fuel injection) or mixture (for carburetor)

Key requirements: Extremely high sealing performance to prevent leakage from the crankcase. Reasonable length and volume to optimize intake efficiency and resonance.

Working pressure: Internal is pulsating negative pressure (during crankcase suction) Usually negative pressure (for naturally aspirated) or positive pressure (for supercharged)

Core associated component: Diaphragm valve

Most modern two-stroke engines have a diaphragm valve integrated within or at one end of the intake manifold. This is one of the core advancements in two-stroke technology.

· Function: Acts like a one-way check valve.

· When the piston moves upward and the crankcase creates a vacuum, the spring flap opens, allowing the mixture to be drawn into the crankcase.

· When the piston moves downward and compresses the mixture in the crankcase, the pressure causes the spring flap to close instantly, preventing the mixture from spilling back into the carburetor.

· Significance: Significantly improves low-speed torque and fuel consumption, and prevents "backfire" losses.

Design Key Points and Common Issues

1. Materials:

· Aluminum alloy: Good heat dissipation, high strength, commonly used in high-performance engines.

· Rubber/composite materials: Have elasticity and shock absorption properties, can absorb engine vibrations and prevent carburetor resonance. Low cost, commonly found in general-purpose engines (mowers, chainsaws, etc.).

2. Length and Diameter:

· Will affect the intake resonance effect, thereby changing the power characteristics of the engine. During modification, changing the length of the manifold is one of the methods to adjust the power output curve.

3. Common Faults:

· Air leakage: This is the most common problem. Aging and cracking of rubber parts, damage to the mounting gasket, loose fastening screws, etc. can all lead to air leakage. Symptoms include high idle speed, idling, and weak acceleration.

· Spring valve failure: Broken, deformed or poorly sealed spring valve, resulting in severe power loss, difficult starting, and re-stalling.