by Jim Kerr
The basic four-stroke automobile engine still operates in much the same manner as it did when it was first designed over 100 years ago. The force of the burning fuel pushes a piston down a cylinder; a connecting rod hinged at the piston transfers this push to the crankshaft; and the crankshaft rotates to drive the rest of the vehicle’s powertrain.
While the basic principles have not changed, the engine certainly has. Even in the 1970’s, it was considered normal for some engines to have major work done at 70 or 80 thousand kilometres. Now engines typically will operate for 160,000 kilometres without major repairs and many will go 300,000 kilometres with regular maintenance!
Not only are the engines more durable; they are lighter, more fuel efficient, and produce more horsepower and lower emissions than ever before. In 1957, a horsepower milestone was achieved for production vehicles when the Chevrolet fuel injected 283 CID engine produced 283 horsepower; one horsepower for every cubic inch. Peanuts compared to
today’s engines! The 2000 Honda S2000 sports car has a 2.0 litre engine producing 240 horsepower: the highest horsepower per litre of any normally aspirated production engine in the world and almost double per litre of that 1957 Chevy 283. Even with such high horsepower, the engine is reliable, economical, idles smoothly and meets low emission vehicle
Much of the increase in engine efficiency can be attributed to changes in cylinder head design. Many engines are produced today with four valves per cylinder. Some even have five! A four valve engine uses two intake valves and two exhaust valves. The valves must be smaller to fit in the engine but this has some advantages. Smaller valves are lighter and can change direction much quicker. This enables the engine to turn faster. A small, high revolution per minute (RPM) engine can produce the same horsepower as a larger, low rpm engine. The small valves and stiff valve train of the Honda S2000 engine allow it to spin all the way to 9000 rpm and that is one of it’s secrets to producing horsepower!
Another advantage of using more valves is better engine breathing. Four small valves can permit more intake mixture and exhaust gas flow when compared to a typical 2-valve engine. The biggest disadvantages of four valve engines are the costly and numerous parts required to build them.
The heart of any engine is the camshaft. The camshaft can be described as a shaft with several egg shaped bumps on it. As the camshaft rotates in the engine, it pushes the valve mechanism to open and close the valves. Some engines have the camshaft in the engine block, but many current designs use overhead camshafts mounted in the cylinder head.
Both Single Overhead Camshafts (SOHC) and Double Overhead Camshaft (DOHC) designs are in common use. The DOHC design uses one camshaft to operate the intake valves and another camshaft to operate the exhaust valves. Cylinder head airflow and engine performance can be increased by the valve arrangement permitted with DOHC designs.
The timing of valve opening is critical to engine performance. Camshaft designs that hold the valves open for only a short time will let the engine idle smoothly but have poor high speed performance. Holding the valves open longer increases high speed engine performance, but also results in rough engine idle and poor low speed performance. Some manufacturers use special drive mechanisms to change the point at which the valves open and close. This is called variable valve timing. It provides both excellent low speed driveability and outstanding high speed performance.
Other parts of the engine are undergoing changes as well. Plastics are replacing aluminum and cast iron as the material of choice for some parts. The plastic is lighter, can be moulded to closer specifications, and may be stronger than the material it replaces. Valve covers, oil
pans, and intake manifolds are now made of plastic on some cars. Other plastic parts that manufacturers have tested include high stress parts such as connecting rods and engine blocks!
Pistons and piston rings are much different from just ten years ago. Pistons are made with special coatings and shorter sides to reduce engine friction. Low tension piston rings are now common. This type of ring uses the pressure of the burning fuel to seal it against the cylinder during the power stroke. During the other three engine strokes the ring slides easily against the cylinder wall.
Advantages of low tension rings include less wear and better fuel economy because of reduced engine friction. One disadvantage is that even a small amount of cylinder wear doesn’t allow the ring to seal. Some manufacturers set wear limits as low as .013 mm (.0005 in.) or 1/4 the thickness of a human hair! These limits are ten times smaller than just a decade ago.
Computer-aided design of parts and improved materials will lead to many more engine improvements in the future. Manufacturers have stated that the next decade will see dramatic changes in automobiles. While each change in engine design may seem small, when added together they produce an engine that has never had better performance and reliability.