Design promises diesel efficiency from natural gas engines - Technology

Diesel Progress North American Edition, Dec, 2002 by Bill Siuru

It is generally accepted that while natural gas engines offer environmental benefits, they often en come at a cost of significantly reduced efficiency and greater equivalent fuel consumption when compared to diesel engines. Sponsored by the Dept. of Energy's National Renewable Energy Laboratory Southwest Research Institute (SwRI), San Antonio, Texas, has developed a gaseous fuel injection concept called the Fuel Injected Prechamber (FIPC) that promises increased part load and full load efficiency in medium-duty natural gas engines to that of modern diesel engines, while retaining or even increasing the emissions benefits of natural gas engines.

The efficiency penalty for natural gas vs. diesel engines is greatest under part load conditions, primarily due to throttling losses. Throttling at light loads with engines burning homogeneous fuel-air mixtures requires reducing the fuel flow rate. However, eventually the mixture becomes so lean that the lean limit is reached, combustion of the mixture is no longer possible and the engine misfires.

SwRI researchers focused on reducing or eliminating throttling losses by using in-cylinder fuel air-charge stratification. While various methods of charge stratification were considered, the FIPC combustion system was found to be the most practical solution.

The FICP's prechamber is located in the spark plug hole with the spark plug cavity forming the prechamber. The self-contained insert that fits in a slightly modified cylinder head has two holes -- one for the spark plug and the other is the fuel introduction port with the fuel supply line attached to this port.

While divided chamber engine designs were highly developed by the late 1970s, three-way catalysts and exhaust gas recirculation became the preferred way to meet stringent emission requirements. Unlike these previous designs where the prechambers were usually small in size and used as an ignition assist for very lean mixtures and to increase the combustion rate of large bore engines, the SwRI design features a much larger prechamber volume. While originally the prechamber volume was about 19 percent of the clearance volume, when testing showed problems, a smaller prechamber with 9.5 percent of the clearance volume was used. Existing prechamber design methods were found to work well in developing the FIPC engine.

In the FICP design, the combustion system operates the engine at part loads by fueling the prechamber alone and modulating torque by adding fuel like in a naturally aspirated diesel engine. At higher loads, fuel is added to the main chamber to form a homogeneous charge and the prechamber now acts as the ignition source like in a conventional prechamber gasoline engine with load controlled by airflow to the engine. Airflow is modulated by a variable geometry turbocharger. A Garrett VNT4O variable nozzle turbine was used in testing.

The FICP system was tested in a six-cylinder, John Deere 8.1 L CNG engine modified with the prechamber nozzles mounted in each of the modified spark plug holes. A simple automotive-type, natural gas port fuel injector proved capable of providing precise control of fueling in the prechamber and was isolated from the prechamber by a ball-type check valve. The cylinder head was machined to accommodate the prechamber caps, nozzles, and fuel injector blocks.

The engine fuel system and control system were also modified. A fuel rail fed the prechamber injectors and a production fuel metering block was adapted to provide pressure and temperature compensation as well as fuel shutoff capability. Fueling for the main chambers was provided by a proportional metering valve with fuel introduction through the production fuel-air mixer. The throttle was retained on the engine but was held at the 100 percent open position during the testing. An Electronic MicroSystems engine control interface unit (EciU) was used. The EciU is a hybrid system that uses a microcontroller to interface with the engine's sensors and actuators in conjunction with a PC for performing the necessary control calculations.

The prototype FIPC engine demonstrated that part load and full load fuel economy improvements could be achieved on medium-duty natural gas engines. Testing showed about a 17 percent reduction in fuel consumption at idle compared to an advanced natural gas engine, a similar 8.1 L open chamber engine. This was accomplished while improving the emissions characteristics of the engine. Indeed, the [NO.sub.x] vs. efficiency tradeoff for the FIPC engine was improved compared to the baseline natural gas engine, with the improvement particularly significant at higher loads.

A BS (brake specific) [NO.sub.x] level of 1.64 g/kW-hr (1.22 g/bhp-hr) is quite low, and very competitive with existing engines. A BSNMHC level of 1.22 g/kW-hr (0.91 g/bhp-hr) is somewhat high, but can be reduced substantially by using an oxidation catalyst. While not measured, the particulate emissions from the FIPC engine should be less than current engines due to the reduction in throttled operation, so less lubricating oil will be consumed and the resulting particulate emissions should be lower.