The Westport HPDI (high pressure direct injection) system on the ISX closely resembles a diesel injection system. Its biggest advantage: it allows the engine to run without an air throttle, similar to a diesel, and with normal torque, which can be limited by knock in other natural gas engines. It has a special injector that handles both diesel and natural gas, but uses the same piston and combustion chamber as a standard ISX, giving the same compression ratio.
Someone running a Westport LNG truck couldn’t abandon diesel entirely. The system relies on diesel fuel for ignition because natural gas has an extremely low cetane rating and would not ignite under normal compression. No more than 7 percent of the total fuel used – and 5 percent on average – is diesel, however, and you have to refuel the small diesel tank only about every week to 10 days, Burke says.
The gaseous fuel is injected at 4,500 psi at a point in the cycle typical of a diesel, beginning after compression is almost complete. It is this unusual feature that allows the engine to run with normal diesel torque and efficiency. Maximum U.S. ratings for these engines are 450 hp with 1,550 lb.-ft. to 1,750 lb.-ft. in a multi-torque arrangement. An Australian version produces 580 hp and 1,850 lb.-ft.
Compared with a pre-2007 diesel, Westport’s LNG engine reduces NOx by 40 percent to 50 percent and particulate by 80 percent. Greenhouse gases are reduced by 20 percent to 25 percent. The 2007 version marketed today is certified to meet the current EPA rule, complete with a diesel particulate filter and the standard levels of cooled exhaust gas recirculation.
Because natural gas burns so cleanly, extended oil changes may be approved as the company accumulates more experience and does oil analysis, Burke says. Since the gas costs 30 percent to 40 percent less than diesel fuel based on energy content, the engine provides a significant fuel cost savings, he says.
So far, this engine has been sold only to fleets. A program to offer the unusual engine to owner-operators running Class 8 rigs will begin early next year, when Kenworth starts production of trucks equipped with the ISX natural gas engine at its plant in Renton, Washington, says Kenworth spokesman Jeff Parietti. At present, the engines are modified for natural gas elsewhere.
Owner-operators using these vehicles will need to operate close to one of the Clean Energy LNG refueling stations, most of which are near large ports or airports. The ports of Los Angeles and Long Beach will offer subsidies to help finance new vehicles to encourage use of low-emissions, alternative-fuel trucks.
Cummins-Westport also is producing a smaller, 8.9-liter natural-gas engine called the ISL G. It’s classed as medium duty, with up to 320 hp and 1,000 lb.-ft. of torque – more torque than hauled some Class 8 loads in the ’70s. It runs like a car engine, using spark ignition and “stoichiometric” combustion, which means that all the oxygen in the air is used up by the burning fuel. High levels of cooled EGR help lower NOx, but the engine also uses an automotive-style three-way catalyst that reduces hydrocarbons, carbon monoxide and NOx. This engine already meets 2010 diesel standards.
Natural gas provides dollar savings in these engines, too, though their lower efficiency makes the savings less dramatic than in the ISX.
While Volvo Trucks North America neither produces nor markets alternative-fuel engines, the parent company staged an interesting demonstration in Brussels in September. It rolled out seven heavy-duty trucks, each of which used a different renewable biofuel, including biodiesel, biogas, dimethyl ether, synthetic diesel, hydrogen gas and ethanol/methanol.
The point was that Class 8 trucks clearly can run on everything from fermented sugar cane to sewage gas; the problem is that petroleum refining and distribution systems are not designed to provide us with these boutique biofuels.
At the Washington International Renewable Fuels Conference held in early March, Volvo detailed the engine modifications needed for some of the fuels such as ethanol and dimethyl ether. Ethanol and methanol require a cetane improver, jacket water charge air cooling in place of air-to-air, and a 24:1 compression ratio. Dimethyl ether needs a low pressure, common-rail injection system and slightly pressurized tank, but the engine produces more torque than with standard diesel fuel. Mack Trucks showed off a dump truck equipped with a hybrid system powered by a flywheel generator and turbine-generator. The system saves fuel by helping with acceleration and powering the engine accessories more efficiently.