Flat engines, also known as opposed engines are a class of internal-combustion engines. There are like straight engines, except for the fact that the crankshaft is rotated by pistons from both the sides. Therefore, a flat engine has two rows of cylinders placed opposite to each other.
A remarkable advantage of Flat engines is that since the piston action is from two opposite sides, the kind of imbalance that arises in a straight-4 engine does not arise here. Thus, the additional mechanical and economical burden caused by fixing a balance shaft is saved.
Flat engines are flat and wide and have a low center of gravity. In sports cars, since the center of gravity is very crucial to its performance, this engine is preferred.
Flat engines are not just flat but also wide. In cars, the steering lock occupies an area near to the engine. So, the problem that is faced in a conventional layout is that it restricts steering lock.
This is the most basic model of the flat engine. It has two cylinders from opposite sides attached to the crankshaft in the middle. There are two kinds of flat-twin engines. a) those with cylinders across frame and b) those with cylinders across frame. Flat-twin engines have been used by companies like Ford, Toyota and in rear-engined BMW cars.
These are better balanced than other four- cylinder engines, are smoother and have very low secondary imbalance. Volkswagen, Porsche, etc. use flat-four engines. In aviation, Lycoming and OS engines have used this in their air crafts.
A Flat-six engine is the next level of flat four with an additional pair of cylinders. They are smoother than flat four but expensive. Porsche 911 uses a flat-six engine.
The H engine:
When two flat engines are placed one above or beside the other, they look like the letter ‘H’. It is also called the ‘H’ block. Each of the engines in a H engine has a separate crankshaft and they are geared together. In case of using an engine with as many cylinders, the length increases considerably. The ‘H’ block solves this. In air crafts, this is used in power-take-offs. In race cars, there is an advantage of a higher center of gravity because one of the crankshafts is placed above the other and away from the ground. The other reason is that a lot of exhaust pipes run beneath the engines.