Cars come in all shapes, sizes and colors sporting different logos. Whether it’s a two-seater, family car, sports car or a luxury sedan, cars run because of one thing, the Engine. All cars have Internal Combustion Engines (IC Engines). There are many types of IC Engines, and in general cars use 4-stroke engines. This can be either a petrol or diesel engine. This post is aimed at providing a general idea regarding the working of a car engine.

Under the Hood By Ecogarf (Own work) [CC-BY-SA-3.0], via Wikimedia Commons

 

Now let’s take a look at a 4-stroke petrol engine. This four stage combustion cycle is also called the Otto Cycle after Nikolaus Otto who invented it in 1867. Before looking at it’s actual working here are some of the components of the engine and their functions explained briefly:

  • Cylinder : This is where all the action takes place. We classify the engines as V6, V8, I4, W12 etc., where the letters represent the arrangement of the cylinders and the numbers are the number of cylinders in the engine
  • Created by Wapcaplet in Blender. (Uploaded by Frack) [CC-BY-SA-3.0], via Wikimedia Commons

    Engine Cylinder Created by Wapcaplet in Blender. (Uploaded by Frack) [CC-BY-SA-3.0], via Wikimedia Commons

  • Valves : The valves open and close regulating the flow of gases into and out of the cylinder. The inlet valve lets fresh fuel mixture into the cylinder and the exhaust valve allows the exhaust gases to leave the system after combustion. The adjacent picture shows the two valves at the top of the cylinder.
  • Camshaft : The opening and closing of the valves are regulated by the camshaft which is basically a shaft having one or more cams(projections) attached to it. These can be used along with rocker arms, pushrods (Over Head Valve systems) or by themselves in the case of Overhead cams (single or dual) to regulate the valves. The adjacent picture shows Dual Overhead Cams rotating above the valves.
  • Piston : The piston moves up and down within the cylinder. This vertical motion is converted to rotational motion in order to move the car. The upper extreme point that the piston reaches within the cylinder is the Top Dead Center (TDC) while the lower extreme is the Bottom Dead Center (BDC)

 

  • Piston rings : These provide a sliding seal to prevent leakage of the air-fuel mixture or combustion products within the cylinder. It also prevents oil from the sump lubricating the crankshaft from getting  pulled into the cylinder and getting combusted
  • Connecting rod : This connects the piston to the crankshaft
  • Crankshaft : The crankshaft is connected to the pistons of each cylinder in the engine and is rotated by the torque provided to it by the pistons through the connecting rod. The crankshaft in turn connects to the flywheel. In the 4-Stroke engine, there is one power stroke per two revolutions of the crankshaft.
  • Spark Plug : This component provides a spark at the end of the compression stroke to ignite the air-fuel mixture and trigger combustion. The precise timing for release of the spark is achieved using the ignition system
  • Fuel Injector : This sprays fuel into the intake manifold so that a good air-fuel ratio is achieved before the air-fuel mixture enters the cylinder

 

Now that you are familiar with some of the components of the engine we can get into the Otto Cycle which is responsible for cars moving.

4-Stroke Engine By Zephyris (Own work) [GFDL], via Wikimedia Commons

  1. Intake Stroke : The piston moves from the TDC to the BDC in this stroke. The inlet valve is open during this stroke to pull in fresh air-fuel mixture from the intake manifold . The fuel injector introduces the appropriate amount of fuel so that the requires air-fuel ratio is achieved. For a petrol engine, the air-fuel ratio used is 8:1 to 12:1
  2. Compression Stroke : The piston moves from the BDC back to the TDC compressing the fresh air-fuel mixture which entered the cylinder. Both the valves remain closed during this stroke. In general, petrol engines have  a compression ratio of 10:1. This means that the air-fuel mixture that enters through the inlet valve is compressed by a factor of 10
  3. Power Stroke : At the end of the Compression Stroke the spark plug ignites the compressed air-fuel mixture causing it to explode into the Power Stroke. This is the stroke that provides torque to the crankshaft. The piston is pushed from the TDC to the BDC. Both the valves remain closed.
  4. Exhaust Stroke : The exhaust valve is opened during this stroke. The piston moves from the BDC to the TDC pushing all the used up air-fuel mixture, i.e., the products of combustion from the power stroke out of the cylinder through the exhaust valve. This makes it possible to start the entire process again.

 

4-Stroke Diesel Engines are actually very similar to petrol engines in their basic working but they differ greatly in where they are used, their cost, fuel economy etc. First, let’s observe the difference in their working.

  • During the intake stroke in diesel engines, the piston moves down allowing air to come into the cylinder as opposed to air-fuel mixture in petrol engines.
  • The air is the compressed as the piston moves up. A much higher compression ratio is achieved in the diesel engine of around 15:1 to 22:1. This increases pressure and temperature and allows better combustion to take place.
  • Instead of a spark plug, the diesel engine has a direct injection system which sprays the fuel into the cylinder. The fuel enters the cylinder at the end of the compression stroke into an atmosphere of increased pressure and temperature and hence spontaneously combusts to give the power stroke.
  • This is followed by the exhaust stroke where the burned gases are pushed out of the cylinder.

Let’s look at what these differences in the process reflect for the engines overall

  • The diesel engine is heavy and bulkier than the petrol due to the higher compression ratio to be achieved.
  • The initial cost of a diesel engine is higher than a petrol engine
  • But the diesel engine gives greater fuel economy (mileage) than the petrol engine. The greater compression ratio allows for a better combustion process and better utilization of available fuel.

Click here to learn about 2-stroke engines