PARTS OF AN ENGINE
Every engine consists of two basic units - the cylinder block/crankcase which forms the lower section of the engine and the cylinder head, which is the upper section.
Both the cylinder block and the head contain a water jacket - a series of passages through which water circulates to cool the engine. The cylinder block also carries a number of externally mounted components like the starter motor, water pump, oil pump and the distributor.
The cylinder block is usually made out of cast iron but more and more car manufacturers use aluminium as it is lighter and dissipates heat better. However, cast iron has strong advantages over aluminium. It is stiffer and absorbs noise better, but with the growing importance of fuel economy and emissions, there is a trend to shift to aluminium as its light weight spells economy.
Cylinders are usually bored directly from the metal of the block, though if the block is made of aluminium, in some instances the bores are lined with steel to give the necessary durability to cylinder walls.
Pistons and Connecting Rods
Pistons are designed to slide up and down the cylinder but must fit very tightly to stop any of the expanding gases during the combustion process from escaping, with the consequent loss of power. Naturally, the piston would wear very quickly against the cylinder if there was no gap at all. Hence, a slight clearance is allowed between the piston and the cylinder wall. This, however, must be sealed to minimise any gas loss. This is achieved by a number of sprung metal rings fitted to each piston, which push outwards towards the cylinder wall to ensure a gas-tight seal. The rings also stop the lubricating oil from the cylinder bore getting into the combustion chamber.
In a regular mid-size car, each piston may be sliding up and down inside its cylinder as many as 100 times a second. Because of this rapid succession of stops and starts and pressures acting upon a piston, especially during the power stroke, the pistons must be strong yet light in weight. The connecting rods form the vital link between the pistons. The top end of each connecting rod, called the small-end, is fitted inside the piston to the gudgeon pin, which allows the rod to pivot as it moves up and down with the piston. The bottom end of the connecting road, called the big end, is bolted to the crankshaft and follows a circular orbit, while the small end follows the up-and-down movement of the piston. Hence the connecting rod is responsible for converting the linear movement of the pistons into the rotary motion of the crankshaft.
The crankshaft runs longitudinally through the crankcase. It is clamped to the block in several places by semi-circular metal caps, which also contain wear-resistant bearings. The main parts of the crankshaft are the journals, which rotate in and are supported by the main bearings. The crank pins rotate in the big end bearings of the connecting rods, linking them with the pistons. The webs or cranks, which join the journals to the crankpins, are so shaped that they serve as balance weights for smoother running.
The back end of the crankshaft carries the flywheel, a heavy and carefully balanced disc, which smoothens out the engine by maintaining a steady rate of turning. The flywheel has a toothed ring gear which meshes with the starter motor. The flywheel also connects with the transmission or gearbox through the clutch assembly which is bolted to the flywheel’s rear face. The front end of the crankshaft is fitted with a gear wheel that usually drives the camshaft. The crankshaft also provides drives to other ancillaries like the oil pump, water pump, power-steering pump and air-conditioner compressor.
The cylinder head is bolted to the top of the cylinder block and covers the top of the cylinders where the valves are located and the combustion takes place. The head itself carries the valve assembly and the camshaft. The spark plugs, the fuel injectors, and exhaust manifolds take in the air-fuel mixture into the cylinders and direct the exhaust gases from the cylinders to the exhaust outlet. Special passages called ports run from the side of the cylinder head to the valve openings.
In simple terms, the job of the cylinder head is to get air and fuel into the engine, burn it in the combustion chamber at the right time and expel the burned gases afterwards. Cylinder heads are now universally made out of aluminium.
Valves and Valve Operating Gear
The valve operating gear is designed so that it opens and closes each valve at the right point in the four-stroke cycle, gives an adequate opening period to allow free gas flow, and operates quietly and reliably over a wide range of engine speeds. The valve gear includes the intake and exhaust valves, the springs which close them and the camshaft, a revolving shaft with cam lobes that actuate the opening and closing of poppet-type valves, which resemble inverted mushrooms. Camshafts are driven either by a chain or a toothed belt and each type has its own set of benefits. Chains are more compact and can last indefinitely. Toothed belts need no lubrication and are usually cheaper.
Almost all modern engines have their camshafts located in the cylinder head and are called overhead camshafts (OHC), but the way they actuate their valves can vary. In a single overhead camshaft design (SOHC) a single camshaft operates both the inlet and exhaust valves, via fingers or rockers. A double overhead camshaft (DOHC) design uses two camshafts, one to operate the inlet valves and the other to operate the exhaust valves. The advantage of a DOHC layout, though it makes the engine more bulky, is that it is actually less complicated to actuate valves as the camshaft directly sits above the valves. A DOHC layout also leaves a central space for the spark plug, which is the optimal location.
While traditional engines used only two valves per cylinder, there is a trend towards multiple valves. Modern engines use four valves per cylinder (two inlet and two exhaust), which increases the gas flow in and out of the cylinder. This enables the engine to breathe better and improves performance. Some engines have five valves per cylinder but operating all those valves in perfect unison can be an engineering nightmare. However, more valves are not necessarily better and many of the latest Merc engines use only three valves per head (two inlet, one exhaust); the space for the fourth valve is taken up by an extra spark plug.
All cars use fuel injection systems to supply fuel to the engine for combustion. In petrol engines, fuel injection has now completely replaced the carburettor, a mechanical device which could not meter fuel accurately enough to provide the optimal air-fuel ratio over a wide range of operating conditions.
In a fuel injection system, fuel is injected under electronic control either in the inlet port (indirect fuel injection) or directly into the combustion chamber (direct injection). Again, each system has its advantages and disadvantages although there is an increasing trend of moving towards direct injection because it offers better fuel economy.