Improvements To Spark Ignition Engines

Overview

The spark ignition (SI) engine is the dominant passenger car and light truck powerplant in the United States. The theoretical efficiency of the SI engine is:

Efficiency = 1 - l/m-l

where r is the compression ratio and “n” the polytropic expansion coefficient, which is a measure of the way the mixture of air and fuel in the engine expands when heated. For a compression ratio of 10:1, and an n value of 1.26 (which is correct for today’s engines, which require the air-fuel ratio to be stoichiometnri, that is, with precisely enough air to allow complete burning of the fuel), the theoretical efficiency of the engine is 45 percent. This value is not attained in practice, but represents a ceiling against which developments can be compared.

Four major factors limit the efficiency of SI engines. First, the ideal cycle cannot be replicated because combustion is not instantaneous, allowing some fuel to be burned at less than the highest possible pressure, and allowing heat to be lost through the cylinder walls before it can do work. Second, mechanical friction associated with the motion of the piston, crankshaft, and valves consumes a significant fraction of total power. Friction is a stronger function of engine speed than of torque; therefore, efficiency is degraded considerably at light load and high rpm conditions. Third, aerodynamic fictional and pressure losses associated with air flow through the air cleaner, intake manifold and valves, exhaust manifold, silencer, and catalyst are significant, especially at high air flow rates through the engine. Fourth, SI engines reduce their power output by throttling the air flow, which causes additional aerodynamic losses called “pumping losses” that are very high at light loads.

Because of these losses, production spark ignition engines do not attain the theoretical values of efficiency, even at their most efficient operating point. In general, the maximum efficiency point occurs at an engine speed intermediate to idle and maximum rpm, and at a torque level that is 60 to 75 percent of maximum. “On-road” average efficiencies of engines used in cars and light trucks are much lower than peak efficiency, since the engines generally operate at very light loads--when pumping losses are highest--during city driving and steady state cruise on the highway. The high power that these engines are capable of is utilized only during strong accelerations, at very high speeds or when climbing steep grades. And during stop-and-go driving conditions typical of city driving, a substantial amount of time is spent at idle, where efficiency is zero. Typical modem spark ignition engines have an efficiency of about 18 to 20 percent on the city part of the Environmental Protection Agency driving cycle, and about 26 to 28 percent on the highway part of the cycle.

During the 1980s, most automotive engine manufacturers improved engine technology to increase thermodynamic efficiency, reduce pumping loss and decrease mechanical fiction and accessory drive losses. These improvements have resulted in fuel economy benefits of as much as 10 percent in most vehicles.

Increasing Thermodynamic Efficiency

Increasing the thermodynamic efficiency of SI engines can be attained by optimum control of spark timing, by reducing the time it takes for the fuel-air mixture to be fully combusted (burn time), and by increasing the compression ratio.

Spark timing

For a particular combustion chamber, compression ratio and air fuel mixture, there is an optimum level of spark advance for maximizing combustion chamber pressure and, hence, fuel efficiency. This level of spark advance is called MBT for “maximum brake torque. ” Owing to production variability and inherent timing errors in a mechanical ignition timing system, the average value of timing in mechanically controlled engines had to be retarded significantly from the MBT timing so that the fraction of engines with higher than average advance owing to production variability would be protected from knock. The use of electronic controls coupled with magnetic or optical sensors of crankshaft position has reduced the variability of timing between production engines, and also allowed better control during transient engine operation. More recently, engines have been equipped with knock sensors, which are essentially vibration sensors tuned to the frequency of knock. These sensors allow for advancing ignition timing to the point where trace knock occurs, so that timing is optimal for each engine produced regardless of production variability. Manufacturers expect that advanced controls of this sort can provide small benefits to future peak efficiency.

Faster combustion

High-swirl, fast-bum combustion chambers were developed during the 1980s to reduce the time taken for the air fuel mixture to be fully combusted. The shorter the burn time, the more closely the cycle approximates the theoretical Otto cycle with constant volume combustion, and the greater the thermodynamic efficiency. Recent improvements in flow visualization and computational fluid dynamics have allowed the optimization of intake valve, inlet port, and combustion chamber geometry to achieve desired flow characteristics. Typically, these designs have resulted in a 2 to 3 percent improvement in thermodynamic efficiency and fuel economy. The high swirl chambers also allow higher compression ratios and reduced “spark advance” at the same fuel octane number. More important, manufacturers stated that advances in this area are particularly useful in perfecting lean-bum engines.

Increased compression ratios

Compression ratio is limited by fuel octane, and increases in compression ratio depend on how the characteristics of the combustion chamber and the timing of the spark can be tailored to prevent knock, or early detonation of the fuel-air mixture, while maximizing efficiency. Improved electronic control of spark timing and improvements in combustion chamber design are likely to increase compression ratios in the future. In newer engines of the 4-valve dual overhead cam (DOHC) type, the spark plug is placed at the center of the combustion chamber, and the chamber can be made very compact by having a nearly hemispherical shape. Engines incorporating these designs have compression ratios up to 10:1, while still allowing the use of regular octane gasoline. High compression ratios also can increase hydrocarbon emissions from the engines, although this is becoming less of a concern with newer combustion chamber designs. Manufacturers indicated that increases beyond 10:1 are expected to have diminishing benefits in efficiency and fuel economy and compression ratios beyond 12:1 are probably not beneficial, unless fuel octane is raised simultaneously. The use of oxygenates in reformulated gasoline could, however, allow the octane number of regular gasoline to increase in the future.