Monday, May 9, 2011

Engine Design Features

This has already been discussed. The number of cylinders can range from two to twelve, or an even number

2. Cylinder arrangement
An engine can have cylinders which are in-line, horizontally opposed or V-type.

3. Cubic Capacity
This is also referred as piston displacement. It is the size of engine, being the total volume, in liters, of all cylinders. A small passenger car engine could have the capacity of 1.5 liters. While a large passenger or light commercial vehicle could have a capacity of 4 liters or more.

4. Valve mechanism
This relates to whether the engine is an overhead-valve (OHV) or an overhead-camshaft (OHC). It can also relate to the number of valves. Some four-cylinder engines in particular have four valves per cylinder and so are referred to sixteen -valve engines.

5. Number of camshafts
Some in-line engines have a single overhead-camshaft (SOHC). Other in-line engines have two overhead-camshafts and referred to as double overhead-camshaft engines (DOHC). V-type engines have two overhead-camshafts; they can also be designed with four overhead-camshafts.
  • Four overhead-camshafts are referred to as Quad cams.

 6. Type of cooling
Engine can be liquid-cooled or Air-cooled. Almost all passenger cars and light commercial vehicles are liquid-cooled.

7. Operating cycle
The operating cycle of an engine can be a four-stroke or a two-stroke cycle. Passenger and light commercial vehicles have four stroke engines but some larger diesel engines are four strokes.

8. Type of fuel or fuel system
Engines are designed to run on a particular fuel.
This can be petrol, distillate, liquefied petroleum gas (LPG) or natural gas for vehicles. Dual-fuel engines can run on petrol or gas.
Petrol engines fitted with electronic fuel injection systems are often called EFI engines with a carburetor are called carburetor engines.

9. Turbocharging and supercharging
Some engines are fitted with an exhaust driven turbocharger, and so are referred to as turbocharged engines. Other engines have a supercharger that is engine driven. Turbocharging and supercharging enable air to be forced into the cylinder and this increases the engine performance.

Rotary engines
Only some of features above apply to rotary engines. Rotary engines do not have cylinders or valve mechanisms. They have a rotor or rotors instead of pistons and they have their own operating cycle. They operate on petrol and can be turbocharged.
  • Rotary engines are the subject of other chapter that will be discussed further.

Hybrid engines
Hybrid powered vehicles have been developed to provide transport with a low fuel consumption and low exhaust emission. The hybrid power for the vehicle is provided by a combination of a petrol engine and an electric motor. Vehicles with hybrid power can operate on the petrol engine only. On the electric motor only or on the combination of both. When operating on electricity the petrol engine is closed down so that there are no engine emissions.
A general arrangement of hybrid power unit is shown in the fig1.17. The engine compartment contains the petrol engine and the special electric motor. These are connected through a hybrid transmission to the front driving wheels Electric power is provided by a bank of special batteries located in the boot of vehicle. The batteries are charged by the petrol engine while it is running and there is also some regenerative charging by the electric motor when it is not driving and being used for braking.

Fuel cell engines
Fuel cell vehicle are powered by most common element in the universe, hydrogen. Hydrogen is the most simple and lightest of all the elements. It is normally found in conjunction with oxygen as water (H2O) or carbon as hydrocarbon (HC).
A fuel cell is an electrochemical conversion device, which is similar to a storage battery but operates very differently. Electric current produced through this process drives the electric motors at the wheels. There is no combustion and therefore, no harmful emissions or reliance on fossil fuels. The fuel cell in figure1.18 has in its centre a proton exchange membrance (PEM) coated on either side by a catalyst The PEM is sandwiched on one side of an anode (+ve) electrode and on the other by a cathode (-ve) electrode. When hydrogen lows onto the anode, protons are allowed to pass through the membrance while the electrons are not. The electrons are made to take a different path to get to another side, therefore, producing electric current. A number of fuel cells are put together to form a stack.

Automotive engines
The engine and components described so far are representative of those used in passenger and light and commercial vehicles, but there are others. All manufacturers have a wide range of engines, and while the basic engine construction and principled remain the same. There are many changes in design that occurs as a result of advances in technology.
Representative engines can be seen in figures 1.19 and 1.20. These can be used to identify engine design and features.

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