Other Strategies To Improve Part Load Fuel Economy

This section describes other methods of improving part load fuel economy that have been the subject of considerable investigation. In particular, exhaust gas recirculation (EGR) systems, gasoline direct injection (GDI) and cylinder deactivation are all important strategies on modern automotive SI engines.
As with the rest of the articles in this series, relevance is also given to the use of these techniques within a throttleless load control strategy.

Cylinder Deactivation

When an internal combustion engine is operating at part load, each of its cylinders is only producing a proportion of its maximum work output. It would therefore be possible to achieve the same engine output with a smaller number of cylinders, each working at higher output.
Cylinder deactivation removes some cylinders of an engine from the operating cycle and results in the remaining cylinders operating at a higher load. The increased load on the working cylinders reduces the amount of throttling that is required, with a corresponding drop in pumping losses and hence improved part load fuel economy.
Cylinders are not deactivated individually because the unequal cylinder firing intervals would cause unstable running, hence it is conventional to deactivate groups of cylinders. Typically with a V8 engine, four cylinders would be deactivated so that the engine would operate as a four-cylinder engine with equal firing intervals.
Cylinder deactivation would not normally be employed on an engine with less than six cylinders because of the poor refinement that would result from deactivating cylinders.

Exhaust Gas Recirculation (EGR)

Exhaust gas recirculation (EGR) is the introduction of exhaust gases into the fresh air-fuel mixture prior to combustion.
The presence of exhaust gases in the cylinder occupies cylinder volume that would otherwise be available to fresh charge, allowing the engine output to be restricted with a higher total trapped mass in the cylinder than that for an engine operating without EGR.
Less manifold depression is required to restrict the fresh charge because the cylinder volume available for fresh charge has been reduced by the presence of the exhaust gases and this increase in manifold pressure leads directly to a reduction in pumping losses.
Some EGR is always present in an IC engine because the exhaust gases in the cylinder clearance volume at TDC tend not to be expelled into the exhaust manifold but become mixed with the fresh charge that is drawn into the cylinder during the intake stroke.
The level of EGR is highly dependent upon the valve events occurring around TDC, namely exhaust valve closing timing and intake valve opening timing. Changing the timing of either or both of these valve events using a variable valve actuation system can therefore allow the EGR level to be controlled.
EGR introduced via the valve timing around TDC retains residual gases at a higher temperature than would be achieved if an external EGR system were used to introduce exhaust gases into the intake manifold.
This can lead to detonation problems associated with the increased mean charge temperature. However, set against this possible disadvantage, internally generated EGR offers a reduction in HC emissions that is greater than the proportion of exhaust gas being retained in the cylinder.
This is because the gases that are retained in the cylinder tend to be those that would be exhausted at the end of the exhaust stroke. This portion of the residual gases tends to contain the majority of the gases from crevice volumes in the cylinder which are typically the source of most unburned hydrocarbons.
EGR and throttleless Operation
Throttleless operation using EGR alone to control the engine output cannot be achieved because there is a limit to the amount of EGR that can be used at part load before the combustion process deteriorates. As the EGR level in the mixture is increased, the combustion speed tends to decrease until the process eventually becomes unstable.
The amount of EGR that can be used before combustion becomes unstable decreases as engine load decreases. At light engine loads such as idle, the detrimental effect of EGR upon the stability of the combustion process requires that the EGR level is minimised.
Whilst EGR alone cannot provide throttleless operation, the use of EGR in conjunction with other strategies for reducing pumping losses can still produce significant benefits.

Gasoline Direct Injection (GDI)

There has been a considerable amount of research into the use of GDI to replace conventional port fuel injection during the last 10 years.
Conventional port injected fuel systems aim to mix the incoming fuel and air completely to produce a homogeneous charge. Most if not all current GDI engines are used in a similar manner to accurately control the fuel mass in the cylinder for each cycle, whilst still aiming to create a homogeneous charge by injecting fuel as early as possible in the cycle.
The main area of GDI research however has been the creation of a non-uniform air fuel mixture in the cylinder to allow engine output to be controlled without the need to restrict the air coming in to the cylinder, hence minimising the intake pumping losses.
Ideally, the fuel distribution in the cylinder at part load would maintain an air-fuel ratio of 14.3:1 (stoichiometric) local to the spark plug whilst being surrounded by air that is largely unmixed with the fuel.
In principle, this stratified charge allows engine output to be controlled solely by the quantity of fuel that is injected, whilst air may be drawn into the cylinder unrestricted by any throttle system.
The engine is therefore able to run at very high overall air-fuel ratios, which would not normally result in a combustible mixture, by maintaining a more conventional air-fuel ratio close to the spark plug.
Charge stratification is achieved by injecting the fuel into the cylinder as late in the cycle as possible so that complete mixing of the air and fuel is not possible.
The fuel distribution in the cylinder is controlled either by the surfaces of the piston and the combustion chamber local to the injector spray, by the air motion in the cylinder, or by a combination of both.
Under full load operation, the engine reverts to conventional homogeneous charge operation by injecting fuel early in the cycle.
There is still however a benefit from using GDI at full load due to in-cylinder charge cooling effects, which allows the use of a slightly higher compression ratio.

GDI and throttleless Operation

In practice, the ideal distribution of air and fuel in the combustion chamber at part load is difficult to achieve, particularly across the whole speed and load range of the engine.
Hence, no engine equipped with GDI has completely dispensed with the conventional throttle to date, although significant part load efficiency gains have been demonstrated.

Back To throttleless Operation

Having looked at other part load strategies, the next page looks at potential valve motion strategies for throttleless operation. The ideal valve motion for throttleless operation is compared with practical valve motions and the resulting effects are illustrated on pressure diagrams.
Part 4 – Valve motion strategies