Interference Engines
Encyclopedia
An interference engine is a type of 4-stroke internal combustion piston
engine. Depending on the design of an engine, piston and valve paths may "interfere" with one another as a result of incorrect timing in their movements. (Such designs are also called "interference head" or "non-freewheeling", and include virtually all diesel engine
s. Conversely, non-interfering engines, such as the Mazda B engine
, are called "free-wheeling" or "non-interference" engines.)
In many piston engines using poppet valves
, the valves descend into the combustion area at the top of the cylinder, while the pistons rise into this area from below. If the lowest point of a valve's descent is lower than the highest point that the piston reaches, the engine is considered an interference engine. In normal operation, the relative timing of the valve and piston's motion prevents them from colliding, but if the valve timing is altered through wear or improper adjustment, or if a timing belt breaks or one of the pulleys the timing belt rides on fails, one or more valves may be placed in the path of a piston.
In interference engine designs, regular belt or chain service is especially important as incorrect timing may result in the pistons and valves colliding and causing extensive engine damage and therefore costly repairs. The piston will likely bend the valves or if a piece of valve or piston is broken off within the cylinder
, the broken piece may cause severe damage within the cylinder, possibly affecting the connecting rods. Many manufacturers who were using belts for valve timing have gone back to using chains on new engine offerings, especially on interference designs. However, some non-interference designs have retained belts due to the risk of engine damage from a belt failure being minimal. Some manufacturers liked the belt's quietness compared to the chain, and the ability to make additional profits from routine belt service. However, chains, in many cases, last the life of the engine, rarely requiring maintenance and helping to lower the cost of ownership for car buyers who are conscious of that statistic. Also, it was discovered that the sound difference between the two was negligible. During the peak popularity of the belt, chains or cogwheels were used almost exclusively on overhead valve
(OHV) engines (which rarely are equipped with belts, regardless of the manufacturer and time of design) and almost all overhead camshaft
(OHC) engines received belts. However, chains are lately becoming more popular for OHC designs.
Piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...
engine. Depending on the design of an engine, piston and valve paths may "interfere" with one another as a result of incorrect timing in their movements. (Such designs are also called "interference head" or "non-freewheeling", and include virtually all diesel engine
Diesel engine
A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...
s. Conversely, non-interfering engines, such as the Mazda B engine
Mazda B engine
The Mazda B-series engine is a small-sized, iron-block, inline four-cylinder with belt-driven SOHC and DOHC valvetrain ranging in displacement from 1.1L to 1.8L. It was used from front-wheel drive economy applications to the turbocharged full-time 4WD 323 GTX and rear-wheel drive Miata as well as...
, are called "free-wheeling" or "non-interference" engines.)
In many piston engines using poppet valves
Poppet valve
A poppet valve is a valve consisting of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem. The shaft guides the plug portion by sliding through a valve guide...
, the valves descend into the combustion area at the top of the cylinder, while the pistons rise into this area from below. If the lowest point of a valve's descent is lower than the highest point that the piston reaches, the engine is considered an interference engine. In normal operation, the relative timing of the valve and piston's motion prevents them from colliding, but if the valve timing is altered through wear or improper adjustment, or if a timing belt breaks or one of the pulleys the timing belt rides on fails, one or more valves may be placed in the path of a piston.
In interference engine designs, regular belt or chain service is especially important as incorrect timing may result in the pistons and valves colliding and causing extensive engine damage and therefore costly repairs. The piston will likely bend the valves or if a piece of valve or piston is broken off within the cylinder
Cylinder (engine)
A cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work...
, the broken piece may cause severe damage within the cylinder, possibly affecting the connecting rods. Many manufacturers who were using belts for valve timing have gone back to using chains on new engine offerings, especially on interference designs. However, some non-interference designs have retained belts due to the risk of engine damage from a belt failure being minimal. Some manufacturers liked the belt's quietness compared to the chain, and the ability to make additional profits from routine belt service. However, chains, in many cases, last the life of the engine, rarely requiring maintenance and helping to lower the cost of ownership for car buyers who are conscious of that statistic. Also, it was discovered that the sound difference between the two was negligible. During the peak popularity of the belt, chains or cogwheels were used almost exclusively on overhead valve
Overhead valve
An overhead valve engine, also informally called pushrod engine or I-head engine, is a type of piston engine that places the camshaft within the cylinder block , and uses pushrods or rods to actuate rocker arms above the cylinder...
(OHV) engines (which rarely are equipped with belts, regardless of the manufacturer and time of design) and almost all overhead camshaft
Overhead camshaft
Overhead cam valvetrain configurations place the engine camshaft within the cylinder heads, above the combustion chambers, and drive the valves or lifters in a more direct manner compared to overhead valves and pushrods...
(OHC) engines received belts. However, chains are lately becoming more popular for OHC designs.