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Gasoline engine12 Jun 2019
The efficiency of the charging process in an automotive engine usually rises to a peak of slightly more than 80 percent at about half the rated speed of the engine and then decreases considerably at higher speed. This change in air charge per cycle with engine speed is reflected in proportionate changes in the torque, or turning effort, applied to the crankshaft and causes the power that the engine can deliver at full throttle to reach a maximum as engine speed increases. At speeds above this peaking speed, the air charge introduced per cycle falls off so rapidly that less power is developed than at lower speeds. The inability of the engine to draw in a full charge of fresh air at high speeds limits the power output of the engine.
Supercharging overcomes this disadvantage by using a pump or blower to raise the pressure of the air supplied to the cylinders and increase the weight of charge. The loss in power suffered by unsupercharged engines at high altitudes (e.g., flying or driving over mountains) can be largely restored. It is also possible to more than double the power of an engine by supercharging; however, increased charge density and temperature, resulting from supercharging, increase the tendency for combustion knock or roughness in the spark-ignition engine and thus necessitate an undesirable decrease in compression ratio or the use of an antiknock fuel.
The supercharging blower may be geared to the crankshaft, in which case the power consumed in driving it is added to the friction loss of the engine. A turbocharger employs a gas turbine operated by the exhaust gases to drive a centrifugal blower. The turbocharged engine not only gains increased power capacity but also operates at improved fuel economy. Historically, large airplane reciprocating gasoline engines were usually supercharged both by geared blowers and by turbochargers to provide the large pumping capacity needed at high altitude; however, these engines have generally been replaced by turboprop engines. High-performance general aviation aircraft typically use turbocharged engines.
Since compressing air prior to introducing it into the cylinder increases the charge-air temperature, the mass of air that can be introduced into the engine is less than that which would be possible if the compressed air were at ambient temperature. Consequently, engine charge-air coolers, commonly referred to as either intercoolers or aftercoolers, are used to reduce the temperature of the charge air. Both air-to-coolant and air-to-air type coolers are available.
The cylinders of internal-combustion engines require cooling because of the inability of the engine to convert all of the energy released by combustion into useful work. Liquid cooling is employed in most gasoline engines, whether the engines are for use in automobiles or elsewhere. The liquid is circulated around the cylinders to pick up heat and then through a radiator to dissipate the heat. Usually a thermostat is located in the circulating system to maintain the designed jacket temperature—approximately 88 °C (190 °F). The cooling system is usually pressurized to raise the boiling point of the coolant so that a higher outlet temperature can be maintained to improve thermal efficiency and increase the heat-transfer capacity of the radiator. A pressure cap on the radiator maintains this pressure by valves that open outwardly at the designed pressure and inwardly to prevent a vacuum as the system cools.
Some engines, particularly aviation engines and small units for mowers, chain saws, and other tools, are air-cooled. Air cooling is accomplished by forming thin metal fins on the exterior surfaces of the cylinders to increase the rate of heat transfer by exposing more metal surface to the cooling air. Air is forced to flow rapidly through the spaces between the fins by ducting air toward the engine.
Lubrication is employed to reduce friction by interposing a film between rubbing parts. The lubrication system must continuously replace the film.
The lubricants commonly employed are refined from crude oil after the fuels have been removed. Their viscosities must be appropriate for each engine, and the oil must be suitable for the severity of the operating conditions. Oils are improved with additives that reduce oxidation, inhibit corrosion, and act as detergents to disperse deposit-forming gums and solid contaminants. Motor oils also include an antifoaming agent. Various systems of numbers are used to designate oil viscosity; the lower the number, the lighter the body of the oil. Viscosity must be chosen to match the flow rate of oil through a part to the designed cooling requirements of the part. If the oil is too thick it will not flow through the part fast enough to properly dissipate heat. Certain oils contain additives that oppose their change in viscosity between winter and summer.
Oil filters, if regularly serviced, can remove solid contaminants from crankcase oil, but chemical reactions may form liquids that are corrosive and damaging. Depletion of the additives also limits the useful life of lubricating oils.
The lubrication system is fed by the oil sump that forms the lower enclosure of the engine. Oil is taken from the sump by a pump, usually of the gear type, and is passed through a filter and delivered under pressure to a system of passages or channels drilled through the engine. Virtually all modern engines use full-flow type oil filters. Filtered oil is supplied under pressure to crankshaft and camshaft main bearings. Adjacent crank throws are drilled to enable the oil to flow from the supply at the main bearings to the crankpins. Leaking oil from all of the crankshaft bearings is sprayed on the cylinder walls, cams, and up into the pistons to lubricate the piston pins. Additional passages intersect the cam-follower openings and supply oil to hydraulic valve lifters when used. A spring-loaded pressure-relief valve maintains the pressure at the proper level. Oil is important for both lubrication and cooling.
Combustion products exit the engine cylinder through the exhaust valves in the cylinder head. Engines may be configured with either an exhaust manifold or an exhaust header. The exhaust manifold is a common chamber to which all the cylinders directly feed combustion products. The advantages of this method are manufacturing and positioning simplicity. The disadvantage is irregular backpressure at the exhaust ports of the cylinders. Headers are composed of a group of tubes, all of common length, connected on one end to each cylinder exhaust-valve location and on the other end to a common exit throat.
The exhaust gases in modern automotive engines next pass through an emission-control device. Emission-control sensors and catalytic converters for reducing air pollution are additional exhaust-system components. Typically, exhaust gases enter a catalytic converter to reduce nitric oxide emissions. The next chamber reduces unburned hydrocarbons and carbon monoxide exhaust emissions.
The reactor system for controlling emissions is often composed of a belt-driven air compressor connected to small nozzles installed in the exhaust manifold facing the outlet from each exhaust valve. A small jet of air is thus directed toward the red-hot outflowing combustion products to provide oxygen to consume the hydrocarbons and carbon monoxide. Sensors monitor exhaust-gas parameters (e.g., temperature and oxygen content) and, in electronic fuel-injection systems, provide information to the control unit to assist in reducing pollutant emissions.
Exhaust gases from an internal-combustion engine are passed through a muffler to suppress audible vibrations. When the exhaust valve opens, the pressure in the engine causes an initial gas outflow at explosive velocity. Successive discharges from the cylinders set up pressure pulsations that produce a sharp barking sound. The muffler damps out or absorbs these pulsations so that the gases leave the outlet as a relatively smooth, quiet stream.
Mufflers of early design contained sets of baffles that reversed the flow of the gases or otherwise caused them to follow devious paths so that interference between the pressure waves reduced the pulsations. The mufflers most commonly used in modern motor vehicles employ resonating chambers connected to the passages through which the gases flow. Gas vibrations are set up in each of these chambers at the fundamental frequency determined by its dimensions. These vibrations cancel or absorb those present in the exhaust stream of about the same frequency. Several such chambers, each tuned to one of the predominant frequencies present in the exhaust stream, effectively reduce noise.
Gasoline was originally considered dangerous and was discarded and destroyed at early refineries, which were manufacturing kerosene for lamps. As the gasoline engine developed, gasoline and the engine were harmonized to attain the best possible matching of characteristics. The most important properties of gasoline are its volatility and antiknock quality. Volatility is a measure of the ease of vaporization of gasoline, which is adjusted in the production process to account for seasonal and altitude variations in the local market. Properly formulated gasoline helps engines to start in cold weather and to avoid vapour lock in hot weather.
To suit the needs of a modern engine, a gasoline must have the volatility for which the fuel system of the engine was designed and an antiknock quality sufficient to avoid knock under normal operation. Although other specifications must also be met, volatility and knock rating are the most important. The size and structural arrangement of the molecules principally determine the knocking tendency of a gasoline as well as its volatility.
Tetraethyl lead, added to gasolines for many years to improve antiknock fueling, has been found to contaminate the exhaust gases with poisonous lead oxides, and so the practice has ended. Lower compression ratios and improved combustion-chamber designs have eliminated the need for extremely high-antiknock gasolines.
Lubricating oil is added to gasoline used in crankcase-compression two-stroke-cycle engines.
The performance of an engine is expressed in terms of power, speed, and fuel economy. The three quantities are evaluated with a dynamometer, a laboratory device that applies a controllable load in the form of resistance to the turning of the crankshaft and also measures the torque exerted at the shaft coupling. The resistance imposed by a dynamometer may be adjusted so that the desired engine speed is established at any throttle position. It is thus possible to run the engine at various speeds throughout its operating range, to continuously maintain these operating conditions, and to measure the precise load and speed at which each run is made. Additional test equipment permits measurement of the exact quantity of fuel consumed, as well as the duration of the runs. From these data the power-speed-economy relationships can be calculated and performance plotted.
The power produced by an engine is expressed in horsepower. When the power developed is measured by means of a dynamometer or similar braking device, it is called brake horsepower. This is the power actually delivered by the engine and is therefore the capacity of the engine. The power developed in the combustion chambers of the engine is greater than the delivered power because of friction and other mechanical losses. This power loss, called the friction horsepower, can be evaluated by “motoring” the engine (driving it in a forward direction) with a suitable dynamometer when no fuel is being burned. The power developed in the cylinder can then be found by adding the friction horsepower to the brake horsepower. This quantity is the indicated horsepower of the engine, so called from an instrument known as the engine indicator, which is used to measure the pressure on the piston and thus calculate the power developed in the cylinder.
Mechanical efficiency is defined as brake horsepower in percent of indicated horsepower and is usually between 70 and 90 percent for normal operating speeds.
A quantity called brake mean effective pressure is obtained by multiplying the mean effective pressure of an engine by its mechanical efficiency. This is a commonly used index expressing the ability of the engine, per unit of cylinder bore, to develop both useful pressure in the cylinders and delivery power. If the power delivered is increased by any change other than an increase in speed or cylinder dimensions, its brake mean effective pressure increases proportionately.
Comparison with other engines
When the gasoline engine is compared with other types of internal-combustion engines, certain similarities and differences, as well as some advantages and disadvantages, become apparent. The diesel engine and the gas engine (an engine utilizing a gas such as compressed natural gas or propane as the fuel) have a good deal in common with the gasoline engine, since they are all cylinder-and-piston engines that burn air-fuel mixtures in contact with moving components. The important difference that distinguishes the diesel engine is that it has no spark-ignition system. Compared with a gasoline engine of the same horsepower, the diesel engine is heavier and more expensive, but it has a longer life and operates at less cost per horsepower-hour because it burns less fuel.
The gas engine has much in common with the gasoline engine; in fact, in some instances their differences are very slight at best. Structurally, the difference lies primarily in the substitution of a gas-mixing valve for a carburetor. The cylinder and piston configurations are the same. In general, gases have better antiknock qualities than gasoline, permitting slightly higher compression ratios without knock or other combustion difficulties.
From the standpoint of application, the gas engine burning natural gas, manufactured gas, or industrial by-product gas is limited primarily to stationary power plant use because it must remain connected to the gas pipeline. If, however, the fuel is liquefied petroleum gas, sometimes called bottled gas, the containers of gas can be carried in a vehicle, leading to much flexibility in applications. The present obstacle is that facilities are not readily available for replenishing the gas supply.