Otto Cycle Operation with Direct Injection

The theoretical Otto Cycle process is the same for both indirect and direct fuel injection methods, but the efficiences gained by using direct injection are bringing the practical application closer to the theoretical.

Direct injection means that there is a total separation between the air and fuel required for combustion. This allows precise control over the quantity of fuel and the time in the cycle it is introduced into the cylinder. Thus, for maximum power (in similar manner to that of a port injection system), it is possible to inject a full quantity of fuel in the induction stroke, while for low load, maximum economy (lean-burn) operation it is possible to inject a smaller quantity of fuel during the compression stroke.

Although lean-burn is implemented with indirect injection, the lean-burn misfire limit (point at which misfire occurs) is governed by the leaness of the fuel/air mixture in the cylinder. This limit is lowered in direct injection, spark ignition engines, as the fuel spray is directed towards the sparking plug to ensure that there is a chemically adequate mixture around the plug when the spark occurs.

Run Otto Cycle with LTC Animation

Induction Stroke

The induction stroke is generally considered to be the first stroke of the 4-stroke cycle. At this point in the cycle, the inlet valve is open and the exhaust valve is closed. As the piston travels down the cylinder, air is drawn through the inlet port into the cylinder and fuel is directly injected into the cylinder. (For lean-burn operation fuel is injected during the compression stroke - see below.)

From a theoretical perspective, each of the strokes in the cycle complete at Top Dead Centre (TDC) or Bottom Dead Centre (BDC), but in practicality, in order to overcome mechanical valve delays and the inertia of the charge air, and to take advantage of the momentum of the exhaust gases, each of the strokes invariably begin and end outside the 0, 180, 360, 540 and 720 (0) degree crank positions (see valve timing chart).

Compression Stroke

The compression stroke begins as the inlet valve closes and the piston is driven upwards in the cylinder bore by the momentum of the crankshaft and flywheel.

Fuel will normally have been directly injected into the cylinder during the induction stroke, but as described above to achieve lean-burn economies, this may be delayed until late in the compression stroke.

Spark Ignition

Spark ignition is the point at which the spark is generated at the sparking plug and is an essential difference between the Otto and Diesel cycles. It may also be considered as the beginning of the power stroke. It is shown here to illustrate that due to flame propagation delays, spark ignition timing commonly takes place 10 degress before TDC during idle and will advance to some 30 or so degrees under normal running conditions.

Power Stroke

The power stroke begins as the fuel/air mixture is ignited by the spark. The rapidly burning mixture attempting to expand within the cylinder walls, generates a high pressure which forces the piston down the cylinder bore. The linear motion of the piston is converted into rotary motion through the crankshaft. The rotational energy is imparted as momentum to the flywheel which not only provides power for the end use, but also overcomes the work of compression and mechanical losses incurred in the cycle (valve opening and closing, alternator, fuel pump, water pump, etc.).

Exhaust Stroke

The exhaust stroke is as critical to the smooth and efficient operation of the engine as that of induction. As the name suggests, it's the stroke during which the gases formed during combustion are ejected from the cylinder. This needs to be as complete a process as possible, as any remaining gases displace an equivalent volume of the new charge of fuel/air mixture and leads to a reduction in the maximum possible power.

Tuned exhaust manifolds help to maintain the momentum of the gas during the stroke to assist in the removal of the exhaust gases. They can also be tuned within the maximum power rev range to create reflections or standing waves at the exhaust port to prevent some of the charge air from disappearing through the exhaust port during valve overlap (see below).

Exhaust and Inlet Valve Overlap

Exhaust and inlet valve overlap is the transition between the exhaust and inlet strokes and is a practical necessity for the efficient running of any internal combustion engine. Given the constraints imposed by the operation of mechanical valves and the inertia of the air in the inlet manifold, it is necessary to begin opening the inlet valve before the piston reaches Top Dead Centre (TDC) on the exhaust stroke.

Likewise, in order to effectively remove all of the combustion gases, the exhaust valve remains open until after TDC. Thus, there is a point in each full cycle when both exhaust and inlet valves are open. The number of degrees over which this occurs and the proportional split across TDC is very much dependent on the engine design and the speed at which it operates.