California would require that NOx emissions from medium- and heavy-duty trucks be cut by about 90 percent relative to today’s cleanest diesel. India and China mostly do not use diesel cleanup systems, so their NOx emissions are about 10 times higher. China and India would need to reduce diesel mission by about 98 percent to reach the future California level.
In the United States, some trucks have begun to meet the expected strict NOx limits using large spark-ignition (SI) engines fueled by natural gas. This can probably not be scaled up. Storing and distributing a gaseous fuel raises vehicle cost and poses infrastructure challenges. Using of natural gas can lead to climate impact because of the leakage of methane.
Cohn and Bromberg decided to pursue another approach: a heavy-duty SI engine fueled instead by gasoline. In general, gasoline SI engines produce low NOx emissions. Guided by their computer models, Cohn and Bromberg took a series of steps to increase the power and efficiency of that design without sacrificing its emissions benefits.
Alcohol used to prevent engine knock
The need to prevent knock has up to now limited improvements in efficiency and performance that would be needed for gasoline engines to compete with diesels. Knock causes a metallic clanging noise and can damage the engine. Cohn and Bromberg dealt with the knock problem using alcohol. When the SI engine is working hard and knock would otherwise occur, a small amount of ethanol or methanol is injected into the hot combustion chamber, where it quickly vaporizes, cooling the fuel and air and making spontaneous combustion much less likely. Alcohol’s chemical composition gives inherent knock resistance higher than that of gasoline. The alcohol can be stored in a small, separate fuel tank — as exhaust-cleanup fluid is stored in a diesel engine vehicle. Alternatively, it could be provided by onboard separation of alcohol from gasoline in the regular fuel tank.
Turbocharging used to make smaller engines with the same power and higher compression ratio to boost power
With concern about knock removed, the researchers were able to take full advantage of two techniques used in today’s passenger cars. First, they used turbocharging, but at higher levels. Turbocharging involves compressing the incoming air so that more molecules of air and fuel fit inside the cylinder. The result is that a given power output can be achieved using a smaller total cylinder volume. And second, they used a high compression ratio, which is the ratio of the volume of the combustion chamber before compression to the volume after. At a higher compression ratio, the burning gases expand more in each cycle, so more energy is delivered for a given amount of fuel.
Used three way catalyst to clean up exhaust
They assumed that the mixture of air and fuel inside their engine contained just enough air to burn up all the fuel — no more, no less. That stoichiometric operation permitted important changes not possible in the diesel, which must run with lots of extra air to control emissions. With stoichiometric operation, they could utilize a three-way catalyst to clean up the engine exhaust. A relatively inexpensive system, the three-way catalyst removes NOx, carbon monoxide, and unburned hydrocarbons from engine exhaust and is key to the low NOx achieved in today’s SI engines.
Gasoline-alcohol engine can hit clean operation targest at half the cost
The low-cost three-way catalyst and smaller overall size help make the gasoline-alcohol engine less expensive than the cleanest diesel engine with a state-of-the-art exhaust-cleanup system. Indeed, according to the researchers’ estimates, the cost of the gasoline-alcohol engine plus its exhaust-treatment system would be roughly half that of the cleanest diesel engine.
Long-haul and other heavy-duty trucks, presently almost entirely powered by diesel fuel, face challenges meeting worldwide needs for greatly reducing nitrogen oxide (NOx) emissions. Dual-fuel gasoline-alcohol engines could potentially provide a means to cost-effectively meet this need at large scale in the relatively near term. They could also provide reductions in greenhouse gas emissions. These spark ignition (SI) flexible fuel engines can provide operation over a wide fuel range from mainly gasoline use to 100% alcohol use. The alcohol can be ethanol or methanol. Use of stoichiometric operation and a three-way catalytic converter can reduce NOx by around 90% relative to emissions from diesel engines with state of the art exhaust treatment.Alcohol from a second tank is used to provide increased knock resistance at higher values of torque, enabling high compression ratio, turbocharged operation that provides comparable efficiency and torque to a diesel engine in a smaller size engine. The alcohol can be neat or a high concentration blend. It can also be a hydrous alcohol (alcohol and water). Hydrous alcohol use can reduce the fraction of fuel that must be provided by alcohol by knock suppression through evaporative cooling.We have used computational models to determine minimal alcohol requirements for knock-free operation and to provide illustrative engine parameters for various forms of alcohol and engine operation modes, including upspeeding to reduce alcohol required for knock suppression and use of open-valve port fuel injection to facilitate use of modified SI natural gas or diesel engines as gasoline/alcohol engines.