Spark-ignition engines

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The conventional gasoline engine is characterized by spark-ignition combustion of a homogeneous fuel-air mixture which is stoichiometric over almost the entire operating map. Load control is achieved by adjusting the charge via the intake manifold pressure (quantity control). The intake manifold pressure is controlled by means of a throttle valve and/or the boosting system.

  Schematic design of a gasoline engine piston Copyright: © RWTH Aachen | TME Figure 1: Schematic design of a gasoline engine piston

Combustion chamber geometry: compression ratio & stroke-to-bore ratio

The efficiency of the internal combustion engine is predominantly determined by the basic geometric design of the engine. Consequently, an increase in the compression ratio leads to an increase in efficiency which should be targeted accordingly. However, an excessively high compression ratio in the spark-ignition engine leads to combustion anomalies in the form of knocking combustion. Knocking combustion manifests itself in high-frequency pressure oscillations and high temperatures, which lead to engine damage in a short time. The occurrence of knocking combustion should therefore be avoided at all costs.

  Schematic design of a gasoline engine cylinder head Copyright: © RWTH Aachen | TME Figure 2: Schematic design of a gasoline engine cylinder head

Another basic geometric parameter influencing the efficiency is the stroke-to-bore ratio. A low stroke-to-bore ratio leads to disc-shaped combustion chambers in the region of top dead center. This results in long flame distances, which promote knocking combustion. Furthermore, a low stroke-to-bore ratio results in a high surface-to-volume ratio, which leads to an increase in wall heat losses. A high stroke-to-bore ratio combined with a typical spark-ignition pentroof-shaped combustion chamber also offers the possibility to further increase the compression ratio, since the compression volume in the pentroof can be reduced. In view of these aspects, the development of spark-ignition engines has shown a trend toward increasing stroke-to-bore ratios in recent years.

 

Downsizing & boosting

The reduction of engine displacement, so-called downsizing, in conjunction with boosting of the internal combustion engine has become the favored concept among many vehicle manufacturers in recent years in order to meet the requirements to reduce fuel consumption. To represent a power demand given by the driving situation, the reduction in displacement is compensated for by an increase in the mean effective pressure. The increase in mean effective pressure is realized by an increase in charge pressure via the boosting device. In addition, the increase in mean effective pressure allows for a reduction of the engine speed. Finally, the engine is operated in ranges with low specific fuel consumption. Downsizing and boosting are often combined with direct fuel injection. In this process, the fuel vaporizes directly in the combustion chamber and in return extracts heat from its surroundings. This reduces the combustion chamber temperature, counteracting knocking combustion.

  CAD model of a gasoline engine valve train Copyright: © RWTH Aachen | TME Figure 3: CAD model of a gasoline engine valve train

Variable valve timing

Valve trains with variable timing have been in series production at many vehicle manufacturers for some years now, although the systems differ in terms of their complexity. In the simplest case, a system allows adjustment of the phase position, i.e. the position of the timing is dynamically shifted in the engine map relative to the crankshaft, however, the valve lift is not changed. Somewhat more complex systems also offer the option of valve lift adjustment. Most vehicles with such a system have adjustable camshafts with two different cam profiles per valve. This allows, for example, the use of a small valve lift for low torque requirements and a large valve lift for high torques. The selective deactivation of individual cylinders can also be realized using such a system by switching between cam profiles. In this case, there is a cam lobe with zero lift on the camshaft, i.e. the valve does not open at all with this cam. In the past, a few vehicles had even more complex systems that allowed fully-variable adjustment of the valve lift and valve opening duration. However, these systems are technically very complex and expensive, and so they have not yet become established for the masses.

 

The basic idea behind variable valve timing is to reduce fuel consumption by adjusting the cylinder charge depending on the power demand. By using small valve lifts at low torque requirements, the throttle losses of the engine can be reduced, because the throttle valve has to open further to provide sufficient fresh charge. Another way of reducing fuel consumption is through so-called Miller valve timing. In this case, the time at which the intake valve closes is shifted away from bottom dead center. This results in a reduction in fresh intake gas mass (early intake closing) or partial expulsion of already inducted fresh intake gas mass (late intake closing). The lack of fresh gas mass to achieve the required power is compensated for by increasing the intake manifold pressure. Thus, Miller valve timing can reduce throttle losses at part load and counteract knocking combustion at high load by shifting part of the compression from the cylinder to the boosting unit with downstream intercooler.

  Charge dilution methods Figure 4: Charge dilution methods

Charge dilution: excess air & exhaust gas recirculation

One of the most effective operating strategies for increasing the efficiency of gasoline engines is charge dilution, which has been the subject of a number of research activities in recent years. A distinction can be made between lean-burn operation and charge dilution by exhaust gas recirculation. In lean operation, the excess air ratio is increased above stoichiometry as traditionally used. The fresh charge mixture can be globally homogeneous or stratified. In case of exhaust gas recirculation, the excess air ratio remains in the stoichiometric range. However, part of the cylinder charge consists of exhaust gas which is supplied to the cylinder by means of internal or external exhaust gas recirculation.

 

Both charge dilution concepts enable a reduction in throttle losses at part load due to the increased gas mass that has to be supplied to the cylinder. At high loads, charge dilution leads to a reduction of the combustion temperature, which can counteract knocking combustion. Furthermore, a lower combustion temperature results in lower nitrogen oxide emissions, so that exhaust gas aftertreatment of these emissions might be omitted if necessary. However, the dilution of the fresh mixture is limited by the ignition system, among other things. If the excess air ratio increases too much, a conventional spark-plug can no longer ignite the mixture. Furthermore, the combustion duration increases at high excess air ratios, which has a negative effect on efficiency. To further improve the efficiency of the spark-ignition engine, alternative combustion processes are being developed that reduce the conventional spark-ignition engine losses.

 

Alternative combustion processes

The conventional spark-ignition engine has two major sources of losses. These are firstly the throttle losses at part load and secondly the restrictions due to knocking combustion at high load.

Water injection offers great potential for reducing the knock-tendency, especially if the injection occurs directly into the combustion chamber. The injected water evaporates in the combustion chamber and extracts heat from its surroundings. As a result, the combustion chamber temperature decreases and knocking combustion can be counteracted.

  Schematic overview of a pre-chamber combustion process Copyright: © RWTH Aachen | TME Figure 5: Schematic overview of a pre-chamber combustion process

For some years now, pre-chamber combustion processes have increasingly been the focus of research. Here, the spark-plug is located in a small chamber in the combustion chamber roof, which is connected to the main combustion chamber by means of overflow holes. If only the spark-plug is positioned in the pre-chamber, this is referred to as a passive or unscavenged pre-chamber. If there is an additional injector in the pre-chamber, it is referred to as an active or scavenged pre-chamber. Pre-chamber combustion processes can be operated in stoichiometric and lean conditions and extend the lean-limit compared to conventional ignition with a spark-plug in the main combustion chamber. Furthermore, the combustion duration can be reduced by means of a pre-chamber, thus counteracting knocking combustion.

 

A number of auto-ignition combustion processes have also been developed to extend the lean-burn limit and reduce the combustion duration. These include controlled auto-ignition (CAI), homogeneous charge compression ignition (HCCI) and reactivity controlled compression ignition (RCCI). All processes operate at high excess air ratios and ignition occurs simultaneously at many locations in the combustion chamber towards the end of compression. This results in short combustion durations and low combustion temperatures, which reduces nitrogen oxide emissions.

 

Alternative fuels

Fuels have a significant influence on combustion efficiency and emissions. Alternative fuels offer great potential to further increase the efficiency of the internal combustion engine. Compared to conventional gasoline, alternative fuels offer the possibility of increasing knock-resistance, enthalpy of vaporization and oxygen content. While the knock-resistance and the enthalpy of vaporization contribute to an increase in efficiency, the high oxygen content reduces HC and soot emissions.

  Alternative fuel samples for fundamental experiments Copyright: © RWTH Aachen |FSC Figure 6: Alternative fuel samples for fundamental experiments

The alternative fuels natural gas (Compressed Natural Gas) and liquefied petroleum gas (LPG) are already established automotive fuels. Alcohol fuels such as ethanol are blended with gasoline in small quantities (10 %) and marketed in Germany as E10. However, in other countries the ethanol content of alcohol blended fuels can be as high as 85 %. Ethanol percentages of more than 85 % are referred to as pure alcohol fuels. Methanol and hydrogen are currently being researched intensively as other alternative fuels. These fuels are called e-fuels because they can be produced from renewable energy and thus offer the possibility of a closed-carbon cycle.

 

Exhaust gas aftertreatment

The homogeneous, stoichiometric operation of the conventional spark-ignition engine allows effective and inexpensive exhaust gas aftertreatment of unburned hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOX) by means of a three-way catalytic converter. However, in lean-burn operation, the three-way catalyst can no longer reduce nitrogen oxide emissions due to an oxygen excess. If nitrogen oxide emissions are not below the legislation limits due to the combustion process alone, additional exhaust gas aftertreatment systems are required. NOX storage catalysts or catalysts with selective catalytic reduction (SCR) can be used to reduce nitrogen oxide emissions in lean-burn operation. Furthermore, current legislation limits particulate emissions, so particulate filters are additionally installed in vehicles with gasoline engines.