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Homogeneous Charge Compression Ignition (HCCI) holds great promises for good fuel economy and low emissions of NOX and soot. The concept of HCCI is premixed combustion of a highly diluted mixture. The dilution limits the combustion temperature and thus prevents extensive NOX production. Load is controlled by altering the quality of the charge, rather than the quantity. No throttling together with a high compression ratio to facilitate auto ignition and lean mixtures results in good brake thermal efficiency. However, HCCI also presents challenges like how to control the combustion and how to achieve an acceptable load range. This work is focused on solutions to the latter problem. The high dilution required to avoid NOX production limits the mass of fuel relative to the mass of air or EGR. For a given size of the engine the only way to recover the loss of power due to dilution is to force more mass through the engine.

This paper discusses the effects of cooled EGR on a turbo charged multi cylinder HCCI engine. A six cylinder, 12 liter, Scania D12 truck engine is modified for HCCI operation. It is fitted with port fuel injection of ethanol and n-heptane and cylinder pressure sensors for closed loop combustion control. The effects of EGR are studied in different operating regimes of the engine. During idle, low speed and no load, the focus is on the effects on combustion efficiency, emissions of unburned hydrocarbons and CO. At intermediate load, run without turbocharging to achieve a well defined experiment, combustion efficiency and emissions from incomplete combustion are still of interest. However the effect on NOx and the thermodynamic effect on thermal efficiency, from a different gas composition, are studied as well. At high load and boost pressure the main focus is NOx emissions and the ability to run high mean effective pressure without exceeding the physical constraints of the engine.

Ion current measurements can give information useful for controlling the combustion stability in a multi-cylinder engine. Operation near the dilution limit (air or EGR) can be achieved and it can be optimized individually for the cylinders, resulting in a system with better engine stability for highly diluted mixtures. This method will also compensate for engine wear, e.g. changes in volumetric efficiency and fuel injector characteristics. Especially in a port injected engine, changes in fuel injector characteristics can lead to increased emissions and deteriorated engine performance when operating with a closed-loop lambda control system. One problem using the ion-current signal to control engine stability near the lean limit is the weak signal resulting in low signal to noise ratio. Measurements presented in this paper were made on a turbocharged 9.6 liter six cylinder natural gas engine with port injection.

In this paper a fast NOx model is presented which can be used for engine optimization, aftertreatment control or virtual mapping. A cylinder pressure trace is required as input data. High calculation speed is obtained by using table interpolation to calculate equilibrium temperatures and species concentrations. Test data from a single-cylinder engine and from a complete six-cylinder engine have been used for calibration and validation of the model. The model produces results of good agreement with emission measurements using approximately 50 combustion product zones and a calculation time of one second per engine cycle. Different compression ratios, EGR rates, injection timing, inlet pressures etc. were used in the validation tests.

2D-LDV-measurements were made on the flow from one transfer channel into the cylinder in a small two-stroke SI engine. The LDV measuring volume was located just outside the transfer port. The engine was a carburetted piston-ported crankcase compression chainsaw engine and it was run with wide open throttle at 9000 RPM. The muffler was removed to enable access into the cylinder. No additional seeding was used; the fuel and/or oil was not entirely vaporized as it entered the cylinder. Very high velocities (-275 m/s) were detected in the beginning of the scavenging phase. The horizontal velocity was, during the whole scavenging phase, higher than the vertical.

The objective of this paper is to investigate how the combustion chamber design will influence combustion parameters and emissions in a natural gas SI engine. Ten different geometries were tried on a converted Volvo TD102 engine. For the different combustion chambers emissions and the pressure in the cylinder have been measured. The pressure in the cylinder was then used in a one-zone heat-release model to get different combustion parameters. The engine was operated unthrottled at 1200 rpm with different values of air/fuel ratio and EGR. The air/fuel ratio was varied from stoichiometric to lean limit. EGR values from 0 to 30% at stoichiometric air/fuel ratio were used. The results show a remarkably large difference in the rate of combustion between the chambers. The cycle-to-cycle variations are fairly independent of combustion chamber design as long as there is some squish area and the air and the natural gas are well mixed.

2-D LDV measurements were performed on two different cylinder designs in a fired two-stroke engine running with wide-open throttle at 9000 rpm. The cylinders examined were one with open transfer channels and one with cup handle transfer channels. Optical access to the cylinder was achieved by removing the silencer and thereby gain optical access through the exhaust port. No addition of seeding was made, since the fuel droplets were not entirely vaporized as they entered the cylinder and thus served as seeding. Results show that the loop-scavenging effect was poor with open transfer channels, but clearly detectable with cup handle channels. The RMS-value, “turbulence”, was low close to the transfer ports in both cylinders, but increased rapidly in the middle of the cylinder. The seeding density was used to obtain information about the fuel concentration in the cylinder during scavenging.

Exhaust Emissions from lean burn natural gas engines may not always be as low as the potential permits, especially engines with open loop lambda control. These engines can produce much higher emissions than a comparable diesel engine without exhaust gas after treatment. Even if the engine has closed loop lambda control, emissions are often unacceptably high for future emission regulations. A three way catalyst is, today, the best way to reduce hazardous emissions. The drawback is that the engine has to operate with a stoichiometric mixture and this leads to; higher heat losses, higher pumping work at low to medium loads, higher thermal stress on the engine and higher knock tendency (requiring lower compression ratio, and thus lower brake efficiency). One way to reduce these drawbacks is to dilute the stoichiometric mixture with EGR. This paper compares lean burn operation with operation at stoichiometric conditions diluted with EGR, and using a three way catalyst.

A FeCrAlloy mesh-type catalyst has been used to reduce hydrocarbons (HC) and carbon monoxide (CO) emissions from a 4-stroke HCCI engine. Significant for the HCCI engine is a high compression ratio and lean mixtures, which leads to a high efficiency, low combustion temperatures and thereby low NOx emissions, <5 pmm, but also low exhaust temperatures, around 300°C. It becomes critical to: 1. Ensure that the HCCI-combustion generates as low HC emissions as possible, this can be done by very precise control of engine inlet conditions and, if possible, compression ratio. 2. Ensure that the exhaust temperature is high enough, without loosing efficiency or producing NOx; in order to get an oxidizing catalyst to work. 3. Select proper catalyst material for the catalyst so that the exhaust temperature can be as low as possible.

In an attempt to extend the upper load limit for Homogeneous Charge Compression Ignition (HCCI), supercharging in combination with Exhaust Gas Recirculation (EGR) have been applied. Two different boost pressures were used, 1.1 bar and 1.5 bar. High EGR rates were used in order to reduce the combustion rate. The highest obtained IMEP was 16 bar. This was achieved with the higher boost pressure, at close to stoichiometric conditions and with approximately 50 % EGR. Natural gas was used as the main fuel. In the case with the higher boost pressure, iso-octane was used as pilot fuel, to improve the ignition properties of the mixture. This made it possible to use a lower compression ratio and thereby reducing the maximum cylinder pressure. The tests were performed on a single cylinder engine operated at low speed (1000 rpm). The test engine was equipped with a modified cylinder head, having a Variable Compression Ratio (VCR) mechanism.

The cylinder to cylinder and cycle to cycle variations were measured in a production type Volvo natural gas engine. Cylinder pressure was measured in all six cylinders. Emission measurements were performed individually after all cylinders, and commonly after the turbocharger. Measurements (ECE R49 13-mode) were performed with different spark gap and two different locations for fuel injection, one before the throttle and one before the turbocharger. Heat-release and lambda calculations show substantial cylinder to cylinder variations, due to lambda variations between the cylinders. The slow burn combustion chamber, with low turbulence, results in high cycle to cycle variations (> 100% COV imep) for some of the load cases.

One way to extend the lean burn limit of a natural gas engine is by addition of hydrogen to the primary fuel. This paper presents measurements made on a one cylinder 1.6 liter natural gas engine. Two combustion chambers, one slow and one fast burning, were tested with various amounts of hydrogen (0, 5, 10 and 15 %-vol) added to natural gas. Three operating points were investigated for each combustion chamber and each hydrogen content level; idle, part load (5 bar IMEP) and 13 bar IMEP (simulated turbocharging). Air/fuel ratio was varied between stoichiometric and the lean limit. For each operating point, a range of ignition timings were tested to find maximum brake torque (MBT) and/or knock. Heat-release rate calculations were made in order to assess the influence of hydrogen addition on burn rate. Addition of hydrogen showed an increase in burn rate for both combustion chambers, resulting in more stable combustion close to the lean limit.