The Gas Exchange Process - EngineKnowHow

In the four stroke engine cycle the exhaust stroke removes the combustion products to allow the intake stroke to admit a fresh charge to into the cylinders.   This process is known as the gas exchange process and a measure of how effectively an engine manages this process under full load conditions is known as the volumetric efficiency.

The Exhaust Stroke

The Exhaust Stroke

The Intake Stroke

The Intake Stroke

The gas exchange process is critical as it determines:

 

  • The maximum torque of the engine as torque is largely dependent upon the mass of air trapped in the cylinder.
  • The fuel consumption of an engine as the engine must provide work to draw in the fresh charge and exhaust the combustants, known as the pumping work.
  • The combustion efficiency as combustants which aren’t exhausted will displace the fresh air whilst the inducted air should be highly mixed with the fuel.

 

Boosting is also an important parameter in the gas exchange process as it allows a greater mass of air to be delivered to the cylinder whilst also reducing the pumping work the engine must provide to draw in the fresh charge.

 

Flow through the intake and exhaust system occurs in pulses due to the opening and closing of the valves and these pulses are dampened by hardware in the intake and exhaust systems.

 

Controlling the Gas Exchange Process

Engine: BMWS1000RR, Source: BMW AG

In typical modern 4 stroke engines the gas exchange process is completed by valves which open into the cylinder to allow the fresh charge to enter or the combustants to exit the cylinder.  The timing of these valve events, and how far into the cylinder these valves travel has typically been controlled by camshafts located in the head of the engine which are driven by the crankshaft and rotate causing the lobes on the camshafts to act against the valves and force them open.  Springs are located between the valve head and the camshaft which force the valves to close once the cam lobe isn’t forcing the valves open.

 

The crankshaft, typically via a timing chain or belt, controls the speed of the camshafts.  As the valves only open during the intake and exhaust stroke, which are only two strokes of the four stroke engine cycle the camshaft operates at half the speed of the crankshaft.

Cross Section of Cylinder Head, Source: http://1.bp.blogspot.com/-BqBpB9x8kI4/TouowU9OKMI/AAAAAAAAABQ/BLi7MvMcQH8/s1600/dohc-zylinderkopf-schnitt.jpg;

The main components controlling the gas exchange process are:

Inlet Valves – Valves located at the end of the intake port, which open into the cylinder to allow the fresh charge to engine the cylinder.

 

Exhaust Valves – Valves located at the start of the exhaust port, which open into the cylinder to allow the combustants to flow from the cylinder to the exhaust system.

 

Intake Port – The chamber located between the intake manifold and the cylinder and partly located in each, the intake ports directs the fresh charge into the cylinder.

 

Exhaust Port – The chamber located between the cylinder and the exhaust manifold and partly located in each, the exhaust ports receive the combustants from the cylinder.

 

Intake Manifold – Consists of the intake plenum and the intake runners where the plenum collects the air or air/fuel mixture and distributes the air or air / fuel mixture to the intake runners and then to the intake ports.

 

Exhaust Manifold – Consists of the exhaust runners and collector where the exhaust ports feed the combustants to the runners and onwards to a turbocharger, catalytic converter or the exhaust pipes.

 

Intake Camshaft – The camshaft which opens the intake valves.  The intake and exhaust camshaft can be separate camshafts (Double Overhead Camshaft – DOHC) or a singular camshaft (Single Overhead Camshaft – SOHC).

 

Exhaust Camshaft – The camshaft which opens the exhaust valves.

 

Valve Springs – Springs which force the intake and exhaust valves to close and remain close following their opening by the camshafts.