Internal Combustion Engines. Allan T. Kirkpatrick

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Figure 1.21 A variable valve timing mechanism. (Courtesy of Honda Motor Co.)

      Heavy‐Duty Truck Diesel Engine

A heavy‐duty truck diesel engine is shown in Figures 1.22. This engine is an inline six‐cylinder turbocharged diesel engine with a 137‐mm bore and 165‐mm stroke for a total displacement of 14.6 L. The rated engine power is 373 kW (500 hp). The compression ratio is 16.5 to 1. The engine has electronically controlled, mechanically actuated fuel injectors, and an overhead camshaft. Note that the cylinder head is flat, with the diesel fuel injector mounted in the center of the combustion chamber. The inlet ports impart a swirl to the air in the combustion chamber to improve mixing with the radial fuel spray.

      The top of the piston has a torus‐shaped crater bowl, so that the initial combustion will take place in the piston bowl. The injection nozzles have three to six holes through which the fuel sprays into the piston bowl. The pressure required to spray the diesel fuel into the combustion chamber is of the order of 1000 bar, for adequate spray penetration into the bowl and subsequent atomization of the diesel fuel. The fuel injection pressure is generated by a plunger driven by the camshaft rocker arm.

Figure 1.22 A 5.9 L L6 on‐highway diesel engine. (Courtesy of PriceWebber.)

      Stationary Gas Engine

A stationary natural gas engine is shown in Figures 1.23 and 1.24. Typical applications for stationary engines include co‐generation, powering gas compressors, and power generation. The engine shown in Figure 1.23 is an in‐line eight‐cylinder turbocharged engine, with rated power of 1200 kW, bore of 240 mm, and stroke of 260 mm for a total displacement of 94 L. The compression ratio is 10.9:1. This type of engine is designed to operate at a constant speed condition, typically 1200 rpm. Each cylinder has two intake and two exhaust valves. The piston has a combustion bowl with a deep dish concentrated near the center of the piston, so most of the clearance volume is in the piston bowl.

      Since natural gas engines are operated lean to reduce nitrogen oxides (), prechambers are used to initiate a stable combustion process. Pressurized natural gas is injected into a prechamber above the piston, and a spark plug in the prechamber is used to ignite the natural gas. The increase in pressure projects the burning mixture into the main combustion chamber, where the final stages of the combustion take place. Prechambers are also used in high‐speed diesel engines to achieve acceptable mixing and more complete combustion.

Figure 1.23 A 94 L L8 stationary natural gas engine. (Courtesy of Cooper Energy Services, Inc.)

Figure 1.24 Cutaway view of 94 L L8 stationary natural gas engine. (Courtesy of Cooper Energy Services, Inc.)

1.7 Alternative Powertrain Technology

      In this section, alternative powertrain technology, including electric motors, fuel cells, and gas turbines, are discussed in terms of a particular application where they have some advantage over the internal combustion engine.

      Electric Motors

Electric motors compete with internal combustion engines in the range of powers less than about 500 kW. Driven by the need to adopt low‐carbon technology both for emission reduction and for improvement of outdoor air quality, the pace of change in vehicle electrification has been far faster than envisioned. Vehicle manufacturers are moving from viewing electric motors as a secondary or back‐up source toward adopting electric motors as the primary power source and internal combustion engines as the secondary power source.

      Electric vehicles have a number of advantages over internal combustion vehicles. Electric vehicles are quiet, have lower vibration levels, and cost less to operate, about 1 cent per mile versus 10 cents per mile for internal combustion vehicles. Electric motors have been developed that have high torque‐speed characteristics superior to those of internal combustion engines, and also provide up to 150 kW per wheel. Most of the electric motors currently used in hybrid and electric vehicles are brushless DC motors, with rotor‐mounted permanent magnets. However, use of AC induction motors, in which the rotating magnetic field is produced by electric currents in the stator, is increasing due to their lower cost, and less complex incorporation into the engine powertrain. Electric motor performance maps that contain contours of motor efficiency on a torque‐speed plane are used to choose electric motors for vehicular applications.

      Proponents of electric vehicles point out that almost any fuel, solar photovoltaic panels, or wind turbines can be used to generate the electricity used by an electric vehicle, reducing dependence on fossil fuels. There would be no local fossil fuel exhaust emissions emitted by the electric vehicle in an urban environment. However, if the electricity is generated by a power plant using coal as a fuel, the air pollution generated by the coal power plant would negate the air‐quality advantage of the electric vehicle.

      The main problem with electric vehicles is the batteries used for energy storage. It is generally recognized that a breakthrough in battery technology is required if electric vehicles are to become a significant part of the automotive fleet. Battery packs for vehicles are generally assembled from groups of individual lithium ion batteries, with a total mass of about 3500 kg, and have a life span of about 5 years. The battery pack capacity for automobiles varies from about 25–100 kWh, and fully electric urban buses are equipped with batteries with capacities from 600 to 1000 kWh. The electric vehicles that have been built to date have a limited range of only 100–200 mi (160–320 km), on the order of one‐half of what can be easily realized with a gasoline engine–powered vehicle.

      In addition, the volumetric energy density of a lithium ion battery is currently 0.5–1 MJ/L (150–300 Wh/L), with energy densities of the order of 5 MJ/L under development, significantly lower than gasoline or diesel fuels with energy densities of the order of 35 MJ/L. In cold weather, there is a degradation of battery performance of the order of 15‐30%. A nationwide network of high voltage (240 to 950 V) charging stations is needed to compensate for the limited range of electric vehicles. The charging time for electric car batteries is at least two hours, depending on the charging station voltage, in comparison to a liquid fuel refueling time of the order of a few minutes.

      Hybrid electric vehicles (HEV), which incorporate a small internal combustion engine with an electric motor and storage batteries, have reached the production stage, primarily due to their low fuel consumption and emission levels. A hybrid electric vehicle has an internal combustion engine to provide the energy to meet vehicle range requirements. The battery then provides the additional power needed for acceleration and climbing hills. The battery in an HEV vehicle typically has a capacity of about 50 MJ