Gordon McTaggart-Cowan

Natural gas fuelling for heavy‐duty on‐road use: current trends and future direction

The use of natural gas as an alternative fuel offers the potential for significant benefits, including lower engine‐out emissions compared to conventional fuels. Most in‐use heavy‐duty natural gas engines use a premixed charge of fuel and air which is then ignited by a spark plug. While these systems meet current emissions standards, substantial further reductions are required to meet upcoming regulations. Efficiency penalties due to poor fuel utilization at low load with such premixed charge engines are also a concern. As a result, there is scope for further research into potential improvements to natural gas‐fuelled heavy‐duty engines, especially through direct injection. This work reviews the various alternatives, both in‐use and under development, for fuelling a heavy‐duty engine with natural gas. The emphasis is placed on providing an understanding of the performance of current heavy‐duty natural gas fuelled engines and improvements that future technologies may offer. The need for further fundamental and applied research, both computational and experimental, is also identified.

A supercharged heavy-duty diesel single-cylinder research engine for high-pressure direct injection of natural gas

Abstract A single cylinder of a heavy-duty diesel engine has been commissioned for research on the use of pilot diesel ignited natural gas which is directly injected into the cylinder. The cylinder is supercharged and equipped for exhaust gas recirculation. Performance and emissions measurements have been made over a range of loads, speeds, timings and equivalence ratios to indicate the potential for emissions reduction of high-pressure direct injection of natural gas. With independent control of boost pressure and engine backpressure, an examination has been made of the similarities and differences in performance of the single-cylinder engine and its multi-cylinder counterpart.

Nox. reduction from a heavy-duty diesel engine with direct injection of natural gas and cooled exhaust gas recirculation

Abstract A heavy-duty ISX diesel engine has been commissioned for single-cylinder operation fuelled with pilot diesel ignited natural gas injected directly into the cylinder. The stock ISX engine was modified by replacing the diesel fuelling system with a high-pressure natural gas system, replacing the turbocharger with an independently controlled supercharger and installing a variable-flow exhaust gas recirculation (EGR) system. A study of the impact of cooled EGR on engine performance and gaseous emissions was carried out. Various engine speeds, loads and injection timings were tested over a range of EGR fractions. A preliminary study of the effect of EGR ‘type’—supplemental or replacement—was also carried out. The results indicate that the NOx emissions varied linearly with the intake oxygen mass fraction (representative of the EGR fraction) until NOx emissions reached 20 per cent of their non-EGR levels. Further NOx reductions were achieved with higher EGR fractions, but the rate of reduction was significantly reduced. The NOx reductions were found to be independent of engine speed and load. An overall activation energy for NOx formation was determined by correlating the NOx reductions with a representative flame temperature. The emissions of combustion by-products, including carbon monoxide (CO) and unburned total hydrocarbons (THC) increased significantly at higher EGR fractions. The engine performance and efficiency were not significantly affected except at very high EGR fractions.

Injection Parameter Effects on a Direct Injected, Pilot Ignited, Heavy Duty Natural Gas Engine with EGR

Pilot-ignited direct injection of natural gas fuelling of a heavy-duty compression ignition engine while using recirculated exhaust gas (EGR) has been shown to significantly reduce NO x emissions. To further investigate emissions reductions, the combustion timing, injection pressure, and relative delay between the pilot and main fuel injections were varied over a range of EGR fractions while engine speed, net charge mass, and oxygen equivalence ratio were held constant. PM emissions were reduced by higher injection pressures without significantly affecting NO x at all EGR conditions. By delaying the combustion, NO x was reduced at the expense of increased PM for a given EGR fraction. By reducing the delay between the pilot and main fuel injections at high EGR, PM emissions were substantially reduced at the expense of increased total hydrocarbon (tHC) emissions. In this research, no attempt was made to optimize the injector or combustion chamber for natural gas fuelling with EGR.

Source Apportionment of Particulate Matter from a Diesel Pilot-Ignited Natural Gas Fuelled Heavy Duty DI Engine

In recent years there has been a growing awareness that particulate matter, especially fine diesel particulate, is a health concern. This has stimulated research to develop new technologies to reduce particulate emissions without increasing nitrogen oxide (NO x ) emissions or fuel consumption. Westport Innovations has developed a technology involving high pressure direct injection and combustion of natural gas for medium and heavy-duty engine platforms. At practical compression ratios, the natural gas will not auto-ignite, so a diesel pilot injection is used for ignition. Thus, the soot emissions can have contributions from the combustion of natural gas, diesel pilot, or lubricating oil. While the soot emissions with natural gas as the main fuel are significantly lower than in a conventional diesel engine, it remains important to determine where the soot is coming from to aid in emission reduction strategies. In this study, the contribution of the pilot fuel (a biodiesel blend with higher 14 C content than diesel fuel) was determined using accelerator mass spectrometry (AMS) measurements of 14 C in the exhaust particulate. Results indicate that the pilot fuel contribution to soot ranges from 4-40% over the tested operating conditions; correspondingly, the contribution by natural gas and lubricating oil combined ranges from 60-96%. The highest fraction of soot from the pilot source is at low load without exhaust gas recirculation. The lowest fraction of soot from the pilot source is at high load with exhaust gas recirculation, i.e. the conditions contributing most to mode-averaged emissions.

Development of a Partially-Premixed Combustion Strategy for a Low-Emission, Direct Injection High Efficiency Natural Gas Engine

A heavy-duty engine was modified to operate on natural gas using a partially-premixed charge strategy. The 15 L engine used a production natural gas fuelling system which was capable of providing direct injections of natural gas and diesel at high pressure during the intake stroke and around TDC of the compression stroke. The engine’s compression ratio was reduced to 15.3:1 to maximize load without exceeding the peak cylinder pressure or encountering knock. A multi-mode strategy for the natural gas injection was used: at part-load the injection occurred during the intake stroke, generating a premixed charge, while at high load a second injection was added around TDC to generate a non-premixed combustion phase. Using this strategy, loads up to 19 bar BMEP were achieved with brake efficiencies of nearly 40% and NOx emissions below 0.29 g/kWh. The key parameters needed to achieve the target load without knock were EGR level, premixed EQR, and intake manifold temperature. At high load, smoke emissions were significant, while at part load, high efficiency and low NOx were achieved but unburned fuel emissions increased. CFD simulation results indicated that the part-load barriers were a result of slow flame propagation through the lean premixed mixture. The modelling suggested that methods to overcome this could include partial-premixing and increased turbulence during the later stages of the combustion.Copyright

Pollutant formation in a gaseous-fuelled, direct injection engine

Heavy-duty natural gas engines offer air pollution and energy diversity benefits. However, current homogeneous-charge lean-burn engines suffer from impaired efficiency and high unburned fuel emissions. Direct injection offers the potential of diesel-like efficiencies, but requires further research. To improve understanding of the combustion process and pollutant formation mechanisms in a pilot-ignited, direct injection of natural gas engine with intake charge dilution, the effects of enhanced gaseous jet kinetic energy, gaseous fuel composition (including ethane, propane, hydrogen, and nitrogen), and filtering the recirculated gases were studied. An experimental investigation was carried out on a single-cylinder heavy-duty engine. Fuel consumption, in-cylinder performance and gaseous and particulate emissions (total mass, size distributions, and black carbon content) were measured. The results indicated that increasing the jet kinetic energy significantly reduced particulate matter (PM) emissions due to improved fuel-air mixing, especially at high load. The addition of hydrogen to the fuel reduced emissions of carbon monoxide (CO), unburned fuel (HC) and P M . The largest effects were observed at high load conditions. The addition of ethane and propane to the fuel resulted in increases in P M and CO emissions at all operating conditions tested; no effect on the combustion progression was detected. The addition of nitrogen to the fuel significantly reduced emissions of CO, P M , and HC due to enhancement of the late-cycle combustion event from increased in-cylinder turbulence. Removing P M from the recirculated gases revealed that these particles had no significant effect on the combustion event or on P M emissions. In conclusion, mixing and kinetic enhancement both reduced the gaseous fuel ignition delay. The overall combustion event was, at high load, mixing limited; the combustion rate was unaffected by fuel reactivity but was increased with turbulence enhancement. Emissions formation was found to be a result of multiple influences whose relative importance varied with operating condition. Increased mixing and lower fuel carbon content reduced P M emissions. Reductions in emissions through the addition of hydrogen and nitrogen to the fuel may offer a potential technique to offset increases in emissions due to variations in ethane and propane levels in natural gas.

Particulate Matter Reduction From a Pilot-Ignited, Direct Injection of Natural Gas Engine

This paper reports an evaluation of various combustion strategies aiming to reduce engine-out particulate matter (PM) emissions from a natural-gas fuelled heavy-duty engine. The work is based on a Westport HPDI fuelling system, which provides direct injection of both natural gas and liquid diesel into the combustion chamber of an otherwise unmodified diesel engine. The diesel acts as a pilot to ignite the natural gas, which normally burns in a non-premixed fashion, leading to significant PM formation. The concepts to reduce PM evaluated in this work are: 1) adjusting the relative phasing of the natural gas and diesel injections to allow more premixing of the natural gas prior to ignition; 2) reducing the pilot quantity to increase the ignition delay of the gas jet; and 3) reducing the level of EGR at select modes to reduce PM formation. These strategies are evaluated at steady state using single- and multi-cylinder research engines, supported by CFD analysis. The results demonstrate that allowing limited premixing of the gas jet prior to ignition can significantly reduce PM emissions. Excessive premixing can lead to high rates of pressure rise; EGR can be used to moderate the combustion under these conditions, without causing increased PM emissions. Reducing pilot quantity is another effective technique to reduce PM, primarily by allowing more air to mix with the gas jet before ignition. These various techniques can be combined to form a new operating strategy that significantly reduces engine-out PM and NOx emissions compared to the baseline strategy without significantly impacting fuel consumption.Copyright

Effect of operating condition on particulate matter and nitrogen oxides emissions from a heavy-duty direct injection natural gas engine using cooled exhaust gas recirculation

Abstract Two methods for reducing nitrogen oxides (NOX) emissions from direct injection, compression ignition, heavy-duty engines are exhaust gas recirculation (EGR) and the high-pressure direct injection of natural gas. Tests combining these two techniques were carried out on a single-cylinder research engine (SCRE) based on a modified heavy-duty automotive engine. No attempt was made to optimize the engines combustion chamber or the injector geometry for EGR operation. The SCREs independent charge-air system allowed for more controlled testing over a wider range of test variables than can be carried out by a standard engine. These tests investigated the effects of cooled EGR on particulate matter (PM) and NOX emissions while varying the injection timing, engine speed, equivalence ratio and intake manifold pressure. The results suggested that, with EGR, higher equivalence ratios reduced power-specific NOX but increased PM emissions. Increasing the charge mass at a constant EGR fraction resulted in significant reductions in PM, at the cost of slightly increased NOX By advancing the injection timing at high EGR fractions, PM emissions and fuel efficiency were improved, with only a slight increase in NOX emissions compared to the more retarded injection timings. The engine speed influenced the amount of EGR that could be recirculated, with lower speeds resulting in higher achievable EGR fractions. These results suggest that EGR fractions in excess of 20 per cent can achieve NOX reductions beyond 75 per cent, without causing unacceptable increases in PM emissions or significant reductions in fuel efficiency.

Nitrogen Oxide Production in a Diesel Engine Fueled by Natural Gas

The effect of large exhaust gas re-circulation (EGR) quantities on NOx production in a natural-gas-fueled direct-injection heavy-duty diesel engine has been tested over a range of speed, load, and timing in controlled experiments with a single-cylinder engine. At the highest EGR ratio, as much as 50% of the cylinder-out NOx was NO 2 . NOx results correlated well with oxygen mole fraction in the unburned gas because of the direct dependence of flame temperature on this quantity. Within the range of measurements, speed and load had little or no effect on the relationship between oxygen mole fraction and NOx production. A multi-zone model for estimating combustion rate, flame temperature, wall heat transfer, and NOx production from engine operating conditions and the record of cylinder pressure development with crank angle, was used to interpret experimental measurements. The model showed the incompatibility of test data with the normal form of the extended Zeldovich model on NO production. However a modified form of it served to correlate experimental data with oxygen mole fraction - which was a nearly linear function of flame temperature. The model also served to represent the effects of engine timing (defined here as the crank angle corresponding to 50% cumulative indicated heat release) on NOx production. At highest EGR (lowest oxygen mole fraction) the NOx emissions were of the order of 1 g/kg of fuel. At this condition CO and unburned hydrocarbon emissions were high, indicating the need for enhanced burning rate.