Other Engine And Fuel Technologies

Overview

Numerous engine and fuel technologies have been suggested as powerplants and power sources for the future In general, most of the alternative fuels, with one exception, are hydrocarbon fuels ranging from natural gas to biomass-derived alcohol fuels, and most of these are being used commercially in limited scale in the United States. Although these fuels can offer significant advantages in emissions and small advantages in fuel economy over gasoline/diesel, their properties and benefits have received significant attention over the last decade, and there is a large body of literature on their costs and benefits. The one exception to this is hydrogen, which often is portrayed as the zero emission fuel of the future. Hydrogen’s ability to fuel current and future automobiles is considered in this section.

Alternative engine technologies considered for the future include gas turbine and Stirling engines. (In this context, the two-stroke engine is considered as a “conventional” engine type, as it is similar in operating principles to four-stroke engines). The gas turbine engine, in particular, has received increased attention recently as a power source for hybrid vehicles. As a result, the potential for the gas turbine and Stirling engine in non-traditional applications is also discussed here.

Hydrogen

Hydrogen is viewed by many as the most environmentally benign fuel, because its combustion will produce only water and NOX as exhaust components, and its use in a fuel ceil produces only water as a “waste” product. Because hydrogen, like methanol, must be derived from other naturally occurring compounds at substantial expenditure of energy, fuel economy evaluations of hydrogen vehicles should consider the overall energy efficiency of the hydrogen fuel cycle. Even if hydrogen is produced using electricity from photovoltaic cells, it may be more efficient to use the electricity directly for transportation rather than through the production of hydrogen, depending on the location of the hydrogen production.

Because hydrogen is a gas at normal temperatures and pressures and has very low energy density, it has serious storage problems on-board a vehicle. There are essentially four different ways to store hydrogen, which are as a:

· compressed hydrogen gas,

· cryogenic liquid,

· reacted with metals to form a hydride, and

· adsorbed on carbon sieves.

Compressed hydrogen gas can be stored in high-pressure tanks (of advanced composite material) at pressures of 3,000 to 6,000 pounds per square inch (psi). To store the equivalent of 10 gallons of gasoline, a tank at 3,000 psi must have a volume of 150 gallons, and the tank weight is approximately 200 lbs.137 Doubling the pressure to 6,000 psi does not halve the tank volume because of increasing tank wall thickness and the nonideal gas behavior of hydrogen; at 6,000 psi, the tank volume is 107 gallons, and its weight is 225 lbs. Increasing tank pressure leads to greater safety problems and increased energy loss for compressing the hydrogen; at 6,000 psi, the energy cost of compression is approximately 10 to 15 percent of the fuel energy. Realistically, pressures over 6,000 psi are not considered safe, 138 and tank capacity over 30 or 40 gallons would seriously compromise the room available in a car. Hence, compressed hydrogen gas storage in a car would have the energy equivalent of only about 3.0 gallons of gasoline for a 6,000 psi tank of a size that could be accommodated without seriously impairing trunk room. \

Liquid storage is possible because hydrogen liquefies at -253oC, but a highly insulated--and, thus, heavy and expensive--cryogenic storage tank is required. A state-of-the-art tank designed by BMW accommodates 25 gallons of liquid hydrogen.139 It is insulated by 70 layers of aluminum foil with interlayered fiberglass matting. The weight of the tanks when fill is about 130 lbs, and hydrogen is held at an overpressure of up to 75 psi. The total system volume is about five times that of an energy equivalent gasoline tank (gasoline has 3.8 times the energy content of liquid hydrogen per unit volume), and the weight is twice that of the gasoline tank. Heat leakage results in an evaporation loss of 1 to 2 percent of the tank volume per day. Although the container size for a 120-liter tank would fit into the trunk of most cars, there are safety concerns regarding the venting of hydrogen lost to evaporation, and crash-safety-related concerns.There is also an important sacrifice in overall energy efficiency, because the energy required to liquefy hydrogen is equal to about one-third the energy content of hydrogen.

Metal hydride storage utilizes a process by which metals such as titanium and vanadium react exothermally (that is, the reaction generates heat) with hydrogen to form a hydride. During refueling, heat must be removed when hydrogen is reacting with the metals in the tank; when the vehicle powerplant requires fuel heat must be supplied to release the hydrogen from the tank. For these reasons, the entire tank must be designed as a heat exchanger, with cooling and heating water flow ducts. The hydrogen used must also be very pure, as gaseous impurities impair the chemical reactions in the metal hydride tank Moreover, the weight of metal required to store hydrogen is very high: to store the energy equivalent of 10 gallons of gasoline, the tank would weigh more than 500 lbs.141 The main advantages of the system are safety and low hydrogen pressure. The overall process is so cumbersome, however, that it seems an unlikely prospect for light duty vehicles, although such systems can be used in buses and trucks.

Adsorption in carbon sieves was thought to be a promising idea to increase the capacity of compressed gas cylinders, although there is a weight penalty. However, most recent work on carbon sieves have concluded that the capacity increase is significant only at pressures in the 1,000 to 1,500 psi range; at 3,000 psi or higher pressure, carbon sieves appear to offer no benefit over compressed gas cylinders. 142 Because a pressure of 5,000 psi or more is desirable, it does not appear that this technology is of use for on-board storage .

Hydrogen can be used directly in engines or in fuel cells. When used in conventional IC engines, the combustion properties of hydrogen tend to cause irregular combustion and backfires. 143 To prevent this, BMW has used very lean mixtures successfully, with the added benefit of no measurable emissions of NOX and an improvement in peak energy efficiency of 12 to 14 percent. Because of hydrogen’s low density, however, operating lean results in a power reduction of about 50 percent from the engine’s normal capacity. BMW uses superchargers to restore some of the power loss, 144 but a larger engine is still required, and the added weight and increased fiction losses could offset much of the energy efficiency gain. Mercedes Benz has solved the low power problem by operating at stoichiometry or rich air fuel ratio at high loads, coupled with water injection to reduce backfire and knocking potential. The Mercedes approach results in significant NOX emissions, however, and the engine requires a three-way catalyst to meet ULEV NOX standards. Overall engine efficiency is not much different from gasoline engine efficiency owing to compromises in spark timing and compression ratio.

The use of hydrogen in a compression-ignition (diesel) engine has also been attempted by directly injecting liquid hydrogen into the combustion chamber. Cryogenic injectors operating on low lubricity liquid hydrogen poses difficult engineering problems, however, and auto manufacturers doubt whether a commercially viable system can ever be developed.