Spark ignition engines

Transient Emissions Analysers

Vehicle emissions legislation around the world requires the testing of vehicles against various standard drive cycles, all of which are transient in nature. (Examples include the NEDC, FTP, 10 and 15 mode cycles).

Conventional emissions analysers have time responses ~1 second T10-90%. As an engine undergoes many firing cycles during this time period, and since engines often exhibit significant cyclical variation (even at steady state), fast response emissions analysers are vital tools to calibrate engines for emissions compliance during transient operation.

Spark Ignition Engine Fueling

Spark ignition engines require accurate control of the air/fuel ratio to ensure that the mixture in the cylinder may be ignited by the spark plug, and burn correctly. Air/fuel ratio in gasoline engines is generally run at the stoichiometric ratio to allow the fitting of after-treatment devices such as three way catalysts.

The air and fuel flows in an engine can change rapidly, particularly during engine start, and also during speed / load transients. This leads to various issues, which may be effectively investigated and addressed during development using Cambustion fast response gas analysers.

Crank Fueling

When the engine starts to crank, the ECU injects fuel into the cylinders, often through open inlet valves. Until the engine is able to fire, any injected fuel will ultimately pass out through the exhaust valve. Since the cold catalyst is unable to oxidize the fuel, the unburned HC exits the tailpipe.

Any cold cranking strategy must therefore aim to achieve an ignitable mixture as quickly as possible to avoid very high HC emissions during crank. The major obstacle to this is the poor vaporization of gasoline when the engine is cold. A significant fraction of the gasoline injected fails to vaporize, and remains as a liquid "fuel puddle", either on the walls of the intake manifold in a PFI engine, or inside the cylinder on a GDI engine. GDI engines may use wall or spray guided injection, the latter offering a significant reduction in the tendency of the fuel to puddle on top of the piston during cold start.

The lower the temperature the more pronounced this problem is, so at lower temperatures the ECU must inject additional gasoline to compensate for the failure of all the fuel to vaporize. The Cambustion HFR500 fast response HC analyser can be used to measure HC concentrations during crank, allowing the ECU model for crank fuel to be calibrated and improved.

Download an animation of the hydrocarbons emitted during start and initial idle of a PFI engine.

Fuel Puddle

The first firing cycle of a spark ignition engine is usually lean, since the fuel vaporization is poor. The second firing cycle may then be rich, as the heat from the first firing cycle causes this excess liquid fuel to vaporize.

Once the engine is running, the inlet manifold on a PFI engine still contains a significant volume of liquid fuel- the "fuel puddle." The volume of this puddle will decrease as the temperature of the manifold (which is often actively heated with coolant) increases. When a throttle transient occurs, the airflow may be increased very rapidly by opening the butterfly. The fuel puddle takes longer to respond, and accurate knowledge of the volume and dynamic behavior of the fuel puddle is necessary to ensure that the engine does not undergo large rich / lean excursions during throttle transients.

The HFR500 fast HC analyser has been employed for many years to verify the size of this fuel puddle and also its dynamic behavior, using an injector and spark cut while subsequently motoring the engine to measure the expelled hydrocarbons.

AFR Control

When the engine is cold, the AFR in the cylinder can not be calculated using just injection volume and airflow. Since some of the fuel remains in the liquid phase, the mixture at the plug may be much weaker than that calculated from total fuel injected / total airflow.

A standard UEGO sensor is often fitted to measure AFR, but these sensors give incorrect outputs when HC concentrations are high (which is inevitable during and immediately after cold start).

In this case, the measurement of the products of combustion (CO & CO₂) may be used to infer the AFR of the gas involved in combustion. The Cambustion NDIR500 is a suitable analyser, and since it has a time response of 8ms T10-90% it may be used to calculate the AFR of the gases cycle-by-cycle during engine start and operation.

Alternative Fuels

Alternative fuels such as ethanol or Liquefied Petroleum Gas (LPG) require different calibrations. In particular the vaporization properties of ethanol are very different to gasoline, and this requires different strategies, especially at cold start. The HFR500 is commonly used to calibrate for alternative fuels.

Residuals Modeling

When the engine begins to crank, the lack of manifold vacuum results in good cylinder filling with fresh air. As the vacuum builds up, the proportion of combustion products trapped in the cylinder (internal EGR) rapidly increases. These residuals need to be accounted for in the injection strategy since they reduce the oxygen available for combustion, and the injection volume must be reduced accordingly.

The NDIR500 CO and CO₂ analyser is able to measure the concentration of residual gas inside the cylinder at the end of compression. The fast time response (T10-90% 8ms) allows this calculation to be performed on a cycle by cycle basis, during engine start and normal operation.

This technique is useful for calibrating EGR systems, and also to reveal imbalances in EGR in multi-cylinder engines, which may occur only at certain speed/load points or during transients.

Particulate Emissions

Port fuel injected gasoline engines typically run close to lambda one (to allow the use of three way catalyst aftertreatment). The injection of gasoline onto the back of a hot inlet valve results in good vaporisation of the fuel, and careful chamber and intake design results in good mixture preparation.

As a consequence particulate emissions from PFI gasoline engines are relatively low. There is sufficient oxygen available to fully combust the fuel (overall Air Fuel Ratio) and this oxygen is evenly mixed with the fuel (good mixture preparation).

The adoption of Gasoline Direct Injection (GDI) engines is compelling for fuel economy reasons. Since the gasoline vapour no longer has to enter through the inlet valve, a larger volume of air is able to enter in the same time. This, coupled with the charge cooling effect as the gasoline vapourizes in the cylinder, allows the engine to be downsized for equivalent power. (These advantages effectively result in improved volumetric efficiency). Turbo charging is an attractive option for enhancing power output.

However, the lack of a hot inlet valve to vaporize the gasoline, and the reduced time available, leads to imperfect vaporization and variations in local Air Fuel Ratio. Where a locally rich zone occurs, either through excessive gasoline in vapour form, or through liquid droplets of gasoline which oxidize in a rich condition at their surface, incomplete combustion results in the production of particulates.

The cyclical variation of GDI engines is generally greater than for PFI engines- factors such as impingement of the spray on the piston / cylinder walls can produce very variable results. Future legislation (in Europe) may restrict the permissible particle number emissions from GDI engines (N/km). The legislated technique provides good sensitivity and repeatability, but its comparatively slow response prevents such equipment accuratly resolving transient particulate emissions. It is during speed/load transients (where accurate air and fuel control is necessarily much harder to achieve) where a significant proportion of cycle emissions may occur.

The Cambustion DMS500 offers a faster method of measuring both particle mass and number, which correlates with legislative techniques. With time responses as low as 200ms T10-90% and the ability to sample raw exhaust (not requiring a CVS tunnel) Cambustion's particulate analysers reveal features of engine operation which offer calibrators and developers crucial data to meet emissions legislation.