- Air cleaners
- Carburetor air cleaners
- EFI air cleaners
- Intake manifolds
- Intake air heating
- Volumetric efficiency
- Forced induction
Air cleaners
An air cleaner filters air that passes through it to stop harmful particles reaching the engine.
The air cleaner on a carbureted engine can be on top of the carburetor, or beside the engine, connected to the carburetor by a hose or duct.
Position is usually decided by how much space there is, or bonnet profile.
On some electronically fuel injected engines, the air cleaner is on top of the throttle body, similar to a carburetor.
Other air cleaners are connected by ducts.
Diesel engines often have more than one air cleaner. This may be due to their severe working conditions.
They’re usually mounted away from the engine to obtain cleaner, cooler air.
A lot of air passes through the intake system into the engine. In a gasoline engine, it’s about 15 times the amount of fuel by weight.
By volume that’s 10,000 times more air than fuel.
The air-fuel mixture enters the engine so the air needs to be clean.
Any abrasives that enter the engine can cause wear and damage.
It also has a silencing effect, muffling noise produced by the air entering the engine.
It can act as a flame trap. So if a gasoline engine backfires, the air cleaner can contain the flame within the intake manifold or carburetor.
Carburetor air cleaners
The most common air cleaner is called a dry-element type. It has a replaceable dry-element of pleated paper or cellulose fiber that doesn’t need to be wet or use oil to act as a filter.
The paper is pleated because that gives a much larger surface area in the space.
The element is designed to be fine enough to trap impurities, but porous enough to let air through easily.
Most heavy-duty air-cleaners incorporate a cyclone type pre-cleaner. It collects larger particles from the air before it enters a dry-element cleaner.
It can be mounted directly onto the air-cleaner unit, or the cyclone principle can be incorporated into the main air-cleaner unit.
This dry-element cleaner is fitted close to the engine and it’s linked to the cyclone system by a duct.
The name cyclone comes from angled vanes that give the incoming air a swirling motion. Centrifugal force throws the heavier dirt particles outwards. They can collect in a separate bowl at the bottom of the cleaner.
In the single unit cyclone element, it collects at the end of the air-cleaner case where it can be emptied out manually.
The air then continues into the dry-element air-cleaner.
An efficient cyclone pre-cleaner can remove up to 90% of particles before they reach the main element.
Some older vehicles may use an oil-bath type air cleaner.
The sudden change in direction of the air as it passes over the oil leaves the heavier dust particles to be caught in the oil.
EFI air cleaners
An air cleaner on a multi-point electronic fuel injected engine usually has a different shape from that on a carbureted engine but it serves the same purpose.
In many vehicles, the air cleaner is mounted where it can get cool, clean air. This air is then carried to the throttle body by a long, flexible duct.
Inside the air cleaner, a filter element of pleated paper filters the air and reduces noise.
Some electronically fuel-injected systems have an airflow sensor after the air cleaner element. It accurately measures all air entering the engine and adjusts the air-fuel mixture accordingly. So it’s essential there are no air leaks or it will upset this mixture.
There are also many systems that monitor the mixture by using a closed-loop control.
Intake manifolds
The intake manifold is usually a metal part with several tubular branches, though it can also be made of a special plastic. In carbureted engines, the intake manifold carries the air-fuel mixture into the engine. The cross-sectional area of each tube needs to be kept small to maintain the high air speeds that improve vaporization. At the same time, it cannot be too small, since that restricts the airflow to the engine at higher speeds.
Electronic fuel injected engines with throttle body injection also have intake manifolds that carry air-fuel mixture.
With multi-point injection, the intake manifolds carry air only, so heating of the intake manifold is not needed, and the cross-sectional area of the tubes can be larger. More air can flow and the engine can produce more power. Fuel is injected into the intake ports of the cylinder head.
This cylinder head has intake and exhaust manifolds for a 4-cylinder carbureted engine. It is a crossflow head. That means the intake manifold is on one side and the exhaust manifold is on the other.
On many cars, the intake manifold has a mounting for the carburetor and a flange that bolts onto the cylinder head. It has a branch for each cylinder to carry air-fuel mixture into the combustion chamber.
This intake manifold has a waterjacket under the carburetor mounting. Hot coolant from the cooling system flows through the waterjacket and heats the manifold.
Heating is required in a carbureted engine to provide better vaporization of the air-fuel mixture.
This fuel-injected engine manifold has a plenum chamber that provides a reservoir of air and helps prevent interference with the flow of air between individual branches. It also acts as a silencer. On this primary manifold, the intake pipes are fixed in length.
Other kinds of manifolds have extra valves to change the effective pipe length. They’re computer-controlled by the engine management system to open at a specified engine speed and extend the torque output.
A diesel engine intake manifold carries air only, not fuel. And since no fuel is vaporized in the manifold, it isn’t heated.
The diesel engine doesn’t have a carburetor, therefore has no need for the throttle.
Some diesels use a pneumatic or air-operated governor with a butterfly valve at the entrance to the inlet manifold. This butterfly valve is only used to operate the governor. It is not a throttle butterfly valve as seen on gasoline engines.
Materials
Traditionally, intake manifolds were normally made of light alloy metal castings, however in approximately 70% of modern vehicles the intake manifold are now made from special heat resistant polymers and plastics. Manifolds using this type of construction lighten manifold part weight by up to 50% and contribute to higher fuel efficiency.
These intake manifolds are normally molded from a glass fiber reinforced grade of crystalline polymer that consists of a blend a of syndiotactic polystyrene and polyamide.
The polyamide nylon based materials provide a good solution for these components because of their mechanical properties and their ease of processing during their manufacture.
This type of material is ideally suited for replacing metal in under-the-hood applications because of its strength, stiffness, and chemical resistance under high temperature operating conditions.
In addition, intake manifolds made out of polyamide nylon have succeeded in improving the environmental performance of cars because of the materials ability to be recycled.
Application
The intake manifold has several tubular branches and carries air and or air/fuel mixture from the air cleaner to the inlet valves in the cylinder head.
In carbureted engines, the intake manifold carries the air-fuel mixture into the engine.
The cross-sectional area of each tube needs to be kept small to maintain the high air speeds that improve vaporization. At the same time, it cannot be too small, since that restricts the airflow to the engine at higher speeds.
Electronic fuel injected engines with throttle body injection also have intake manifolds that carry air-fuel mixture. With multi-point injection, the intake manifolds carry air only and the cross-sectional area of the tubes can be larger. More air can flow and the engine can produce more power.Fuel is injected into the intake ports of the cylinder head.
Cylinder heads that have separate intake and exhaust manifolds are know as "crossflow heads". That means the intake manifold is on one side and the exhaust manifold is on the other.
On many cars, the intake manifold has a mounting for the carburetor and a flange that bolts onto the cylinder head. It has a branch for each cylinder to carry air-fuel mixture into the combustion chamber.
Many intake manifolds have a waterjacket under the carburetor mounting. Hot coolant from the cooling system flows through the waterjacket and heats the manifold.
Heating is required in a carbureted engine to provide better vaporization of the air-fuel mixture.
Fuel-injected engine manifolds normally have a plenum chamber that provides a reservoir of air and helps prevent interference with the flow of air between individual branches. It also acts as a silencer.
Normally on primary manifolds, the intake pipes are fixed in length. Other kinds of manifolds have extra valves to change the effective pipe length. They may be computer-controlled by the engine management system to open at a specified engine speed and extend the torque output.
A diesel engine intake manifold carries air only, not fuel. And since no fuel is vaporized in the manifold, it isn’t heated.
Some diesels use a pneumatic or air-operated governor with a butterfly valve at the entrance to the inlet manifold. This butterfly valve is only used to operate the governor. It is not a throttle butterfly valve as seen on gasoline engines.
Intake air heating
Normally, the air entering the air cleaner is cold.
Cooler air is more dense, that is it contains more oxygen, and that should help the fuel burn more efficiently. But fuel doesn’t vaporize very well in cold air.
Since an enormous amount of heat passes through the exhaust manifold,
many carbureted engines capture some of it with a shroud around the exhaust manifold. This heats the incoming air to improve fuel vaporization. This heated air then enters the air cleaner.
Once an engine is hot, it may no longer need the incoming air to be hot. So the amount of hot air entering the engine needs to be controlled.
On vehicles with emission controls, air cleaners use a thermostatic valve to control how much hot air enters the air cleaner.
When the engine is started, only heated air is used. As engine temperature rises, the valve opens, and provides more and more air at normal temperature. This ensures that the temperature of the air supplied to the engine stays fairly constant.
This valve can be a simple thermostatic type too.
Another way to control the amount of hot air is to use a vacuum control unit and a control valve mounted inside the air-cleaner unit.
The vacuum control unit has a diaphragm attached to the control valve.
When a cold engine starts, vacuum from the intake manifold moves the diaphragm. It opens the valve and hot air flows into the air cleaner.
A heat-sensitive valve in the air cleaner responds to changes in air temperature. Below a certain level, it opens, letting hot air flow into the air cleaner. As the temperature rises, it slowly closes, reducing the flow of hot air and blending in cooler air.
Volumetric efficiency
Volumetric efficiency compares the volume of air entering a cylinder during intake, to the internal volume of the cylinder when the piston is at bottom dead centre. It is usually expressed as a percentage.
In a naturally aspirated engine, one without forced induction, volumetric efficiency can never be 100%. This is because of the resistance to the airflow that occurs at the carburetor, manifold, and the intake valve.
With forced induction, the incoming air is compressed, so that a greater volume and mass of air is forced into the cylinder during the intake stroke. That takes the volumetric efficiency to above 100%
Forced induction
One way to improve engine output is to increase the amount of air-fuel mixture that is burned in the cylinder.
That means increased volumetric efficiency, or how much air-fuel mixture is delivered to the engine.
This is done by what is called forced induction.
Forced induction increases air pressure in the intake manifold above atmospheric pressure. An engine using forced induction can have a volumetric efficiency above 100%.
One way to achieve forced induction is by using a turbocharger.
It uses energy that’s normally wasted through the exhaust. Exhaust gases enter a turbine and make it spin. The more gases, the faster it spins.
A shaft connects the turbine to an intake compressor. It compresses the air and forces it under pressure into the intake manifold.
The turbine operates at very high temperatures, and with the compressor can rotate at well over 100,000 revolutions per minute. So they both need a good supply of clean oil to lubricate their bearings and carry heat away.
Some engines also supply coolant to the turbocharger body to improve cooling.
On gasoline engines, higher and higher engine speeds mean more and more exhaust gases, and that makes the turbocharger force more and more air into the cylinders. This can damage the engine.
To control this, a device called a waste-gate is fitted to the exhaust inlet of the turbocharger.
When air pressure in the intake manifold reaches a pre-set level, it automatically directs exhaust gases away from the turbocharger.
The waste gate can also be computer-controlled to reduce intake air pressure in the event of detonation or knocking.
An emergency relief valve may also be fitted so that if the waste gate should fail, it can prevent an abnormal rise in manifold pressure.
Since the turbocharger uses the energy of the exhaust gases, there is a short delay between when a driver opens the throttle and when maximum power is available. This is turbo lag, and on larger engines it’s quite noticeable.
Because a turbocharger recycles heat energy that would otherwise be lost to the engine, it can seem to be offering additional energy for nothing. But a turbocharger can introduce problems of its own.
The extra heat and power it generates can put an extra load on the engine’s cooling and lubrication systems.
As the air passes through the turbocharger, it heats up. But hot air is less dense than cool air, so it tries to expand again and some of the benefits of compressing it are lost.
To stop this and improve efficiency, some engines use an intercooler to cool the compressed air. It fits between the turbocharger and the engine. It’s usually air-cooled.
