OHV engines, also known as pushrod engines, can produce a lot of torque at low engine speeds, but cannot operate at the same high engine speeds as overhead cam engines. They have been widely used by American manufacturers over the years, and OHC stands for overhead camshaft. Valve positions have varied widely throughout the history of valve position development. In the pre-World War II Ford era, flat head valves were the predominant valve configuration.
In short, the valves are placed on the cylinder wall, and the intake and exhaust share the same valve. This kind of structure is simple and convenient, and becomes the choice of mature engine in the age of immature metallurgical technology. However, the congenital defect of poor thermal efficiency has led to its gradual elimination from the market.
However, the vast majority of vehicles are designed for street use. Multi-valve designs (more than two valves per cylinder) have gradually become mainstream to meet low-speed and high-speed performance. Most of these multi-valve engines belong to the OHC structure. The overhead camshaft and overhead valves are better matched to make high and low speed operation a reality.
What is the difference between OHV and OHC engines?
There is no essential difference between OHV and OHC, as their valves appear to be overhead structures at first glance. However, the biggest difference between OHV and OHC is the position of the camshaft.
Compared to the OHC overhead camshaft, the OHV moves the camshaft to the cylinder. In an inline OHV, the camshaft is usually located above or to the side of the crankshaft. In a V-shaped OHV, the camshaft is at a V-shaped angle. Therefore, OHV engines are called I-head engines or pushrod engines.
For the OHV engine, the camshaft must be connected with the push rod to drive the valve to open and close, so the valve structure is more complicated. For the OHC engine, the overhead camshaft directly pushes the valve without a pushrod structure, which greatly simplifies the valve train.
What are the advantages of an overhead valve?
1. Compact structure.
OHV engines are known for their compact construction because the camshafts can be placed at a V-shaped angle. For example, Ford's (OHC) Modular Series V8 has a larger 4.6L overhead camshaft than its predecessor, the 5.0L Windsor Series V8 (OHV).
2. Simplified the drive structure.
For overhead camshaft (OHC) engines, the camshafts need to be driven by a timing belt or chain, and these drives also require tensioners. Therefore, it virtually increases the complexity of the whole system. In contrast, OHV engines only require a short belt or chain, sometimes connected by gears, because the camshaft and crankshaft are closer together. However, this advantage is sometimes offset by complex valve structures.
3. Good for maintenance.
Hydraulic lift arms allow for more efficient adjustments to reduce maintenance effort.
4. Large displacement.
The OHV is designed to deliver a larger displacement in the same space, resulting in greater torque output.
What is the disadvantages of OHV engines?
First, high speed is not welcome among OHV engines.
I think it has a limited speed when it's running. The engine will be destroyed when it run at high speed. Due to the complex valve structure, while the valve opens and closes, the OHV cannot keep up with the breathing rhythm of the engine at high revs. The OHV engine valve might burst or fail to close caused by the high RPM. That's why manufacturers prefer paying more attention to the output at low speeds in the tuning process.
For an OHV engine, the civilian version is usually limited to 6000-7000RPM, while the racing one is around 10500RPM. However, for an OHC overhead camshaft engine, the civilian version can reach 7000-9000RPM, while the racing one is not too difficult to reach 20000RPM.
Second, Loud noice.
An OHV engine is often designed with two valves because it involves a complex series of valve components for the push rod, and the valve shape and size are limited. Besides, the noise of the OHV engine is also much greater than that of OHC.
Third, the multi-valve is actually useless at low speed, and the backward design of the OHV dual valve has a huge advantage at low speed, which can generate large torque.
How does a OHC engine work?
The OHC camshaft is located at the top of the cylinder and is driven from the engine crankshaft by a chain or belt. A cam on the camshaft presses against the rocker arm to turn the valve on and off. After the camshaft leaves the rocker arm, the valve closes due to the action of the spring, and the cam compresses the rocker arm again, and the valve opens. In addition, the VVT, VVL, and other techniques are able to control the length of time of valve opening and closing, opening time, lift, etc.
How does a OHV engine work?
Unlike the OHC, the OHV camshaft may be located in the middle of the engine or below, driven by the crankshaft as well. What’s in contact with the camshaft cam is the cam follower, which can be called a connector or valve tappet that follows the shape of the cam. When the camshaft rotates, the cam forces the tappet to lift, similar to the valve rocker arm. The tappet moves the push rod up and down, and the other end of the push rod pushes against the rocker arm, so that the valve opens. The rocker arm acts like a lever. The tappet pushes against one side and the other side moves with it.