What do they do?
The valves allow the air/fuel mixture into the cylinder, and the exhaust gas out of the cylinder.
The type of valves used in almost all engines are "Poppet Valves". The matching angle of the face and seat, means the more pressure is created in the cylinder, the better the valves seal.
Most engines use one intake valve, and one exhaust valve per cylinder, but an increasing number of high performance engines use four, or even five valves per cylinder. We'll talk about them later.
Cylinder head design, and how fast the fuel/air mixture can flow through the ports, is extremely critical to engine performance. To make more power, an engine must burn more fuel, and the more efficient the engine is at filling its' cylinders with mixture, the more power it will put out. At high speed, the mixture is traveling through the ports at as much as 150 miles per hour, so any restriction will decrease the power output at high RPM. The ports should be as straight as possible, with few corners and protrusions as possible. The ports should also be as large as possible to flow more air. Head design has a great bearing on power.
One of the weakest links in the design of a high performance engine is the type of valves that are used. When you think about it, we are designing a port and manifold as straight as possible, to get as little restriction as possible, and get the fuel/air mixture flowing as quickly as possible, to get as much mixture into the cylinder as possible, so we get as much power as possible, and then we design a system so that when the valve is open, the valve head nicely blocks off the flow of mixture. It seems counter productive. Maybe one day, a better valve system will be invented. Until then, we are stuck with the Poppet Valve system.
The valve stem slides back and forth in the valve guide. The face closes onto the seat, creating the sealing effect. Fuel/air mixture travels through the intake port on its' way into the cylinder , and exhaust gas travels through the exhaust port on its' way out into the exhaust system. The rocker arm, or the lifter pushes on the tip to open the valve. The neck separates the valve's head from its' stem. The margin is the edge of the valve head. The valve springs push on a retainer to keep the valves shut. Spring pressure on the retainer also keeps the locks in place.
In a modern car engine, the valves are located in the cylinder head. They are either overhead valve, or overhead cam. We'll discuss them later, but first, we'll look at the history of the valve train.
Early engines were not capable of making a great deal of power,or turning at high RPM, so port and head design was not that important. The bottom end couldn't take the RPM anyway.
The first car engines had both valves in the block. The intake valve on one side of the cylinder, and the exhaust valve on the other, with separate cams for intake and exhaust. This was called the "T head" engine because the combustion chamber formed a letter "T". It was characterized by low compression, and poor air flow, and therefore low power. The T-head engine was used from the first days of the four stroke, or Otto cycle engine in the 1880's , until the early 1920's.
Early cars had little need for efficient head design, and high horsepower. How fast did you really want to go on unpaved, potholed roads, with wooden spoke wheels, and virtually non-existent brakes on the rear wheels only? None-the-less, as soon as there were two cars on the road, two things happened; they raced; and, they got into an accident. The photo shown below, is from the 1903 Paris-Madrid race. The car is a de Dietrich with a 5,797cc, or 360 cubic inch "T head engine" that produced 30 hp at 1200 RPM. There were cars of that era that had engines as large as 20 liters or 1220 cu.in., and had only 60 hp. They had compression ratios as low as 4:1. Note the chain driven wheels, and riding mechanic. As technology got better, the riding mechanic was no longer needed to fix the car, and became the race driver's second set of eyes. One driver in the 1919 Indianapolis 500, realized that he was carrying around 200 lbs. of extra weight by having a riding mechanic along, just so he could see backwards, and replaced him with a mirror. He won the race, and the next year, all cars had mirrors, and the riding mechanic became history. The rear view mirror has been with us ever since.
L - Head Engines
As fuel technology became better, it was possible for engines to have higher compression ratios.
It was also cheaper to produce an engine with both valves on the same side of the cylinder, since only one cam was needed, so the "L-Head" engine came into being. This engine had its' valves in the block still, but now they were both on the same side of the cylinder. This raised the compression ratio, and therefore the power, by using a smaller combustion chamber. It did, however, cause the mixture to go through a great number of corners, each one a restriction, to get into, and out of, the cylinder. It was first used around 1910. The last car to use an L-Head, or "Flat Head", or "side valve" engine was the Rambler American in 1965. It had 199 cubic inches, and 95 horsepower.
Probably the most famous flathead engine of all time, was the Ford flathead V-8 which was produced from 1932 until 1953. It powered more hot rods during the 1940's and 50's than any other engine. In car use, it only ever made 125 horsepower, but this was in an era when Chevrolet didn't even make a V-8 engine. The hot setup for the flathead, though, was Ardun heads, which were overhead valve.
The Ford flathead had serious design problems; such as overheating. The exhaust gas had to go through the water jackets to get out of the engine, and put a great deal of heat into the coolant.
A picture of the Ford flathead V-8 is below.
Ford flatheads were produced by the millions from 1932 to 1953. The flathead, or "L-head" engine still remains in lawnmower power plants. Both the Tecumseh, and Briggs and Stratton, are "L-head " engines.
For a short period of time from the mid 1950's until the mid 1970's, a few manufacturers, primarily Rolls Royce, Jeep, and Rover, produced engines with their intake valves in the head, and their exhaust valves in the block.
The same camshaft was used for both the intake and exhaust valves, but pushrods and rocker arms operated the intake valves. This design was never very popular, and was superceded by the overhead, or "I-head" design. These engines were characterized by low horsepower, but high torque at low RPM. They were also brutally reliable.
The I-head, or Overhead valve engine has been around for a long time. It has been produced since the 1920's, but its' advantage of small combustion chambers, and therefore high compression, was not realized until fuel technology had advanced enough. This happened during the late 1940's.
Nowadays, all car engines are "I-head" design.