The Motor Vehicle Problem 朱信 Hsin Chu Professor Dept. of Environmental Eng. National Cheng Kung University
1. An Overview of the Problem of Air Pollution from Motor Vehicles • The first gasoline-powered automobiles appeared in 1886. • By 1900 world production was only about 20,000 vehicles per year, compared to about 30 million in 1999. • Although any one car consumes little fuel and emits small amounts of pollutants, together the roughly 500 million of them in the world consume large amounts of fuel and emit large amounts of pollutants.
1.1 Emissions • There are about 123 million autos in the US and about 70 million trucks. • Next slide (Table 13.1)Motor vehicles are the source of three-fouths of the US emissions of CO, and 40 to 50% of the US emissions of HC and NOX.
“Off-road vehicles” in Table 13.1 include aircraft, railroads, boats, construction equipment, and farm equipment. • From Table 13.1, autos and light trucks contribute much more to the US emissions than do these other sources.
1.2 The Regulatory History of Motor Vehicle Air Pollution Control • Motor vehicles did not attract much attention as air pollution sources until about 1950. • As coal combustion sources were controlled, and as natural gas replaced coal as the principal urban heating fuel in the US, a new type of air pollution was discovered in Los Angeles.
A type of eye and nose-irritating air pollutant, later named smog, occurred there, mostly in the summer. • Professor A. J. Haagen-Smit demonstrated the eye-irritating materials were largely formed from emissions from autos.
California began to regulate emissions from autos in 1963. • In the clean Air Act of 1970 US Congress began federal regulation of autos, requiring stricter rules for any states that already had state rules (only California), but also requiring fairly strict rules for the rest of the country. • The history of these regulations is shown in Table 13.2 (next slide).
2. The Internal Combustion (IC) Engine • External combustion engines were developed before internal combustion engines. • James Watt’s 1776 steam engine was the first general-purpose heat engine that converted heat from combustion to a steady flow of power to a rotating shaft.
For 100 years steam engines, with combustion in a boiler external to the power –producing part of the engine, were the only combustion engines. • These steam engines launched a giant technological expansion, which, among other things, led to the development of much better machine tools. • The improved machine tools made it possible to build the first IC engines.
The first commercially successful IC engines (combustion inside the power-producing parts) were those of Otto and Langen about 1876. • For a given power output these engines were substantially smaller and lighter than external combustion engines and had a higher thermal efficiency (lower fuel consumption).
Those features made them the natural choice for motor vehicles. • The steam engine held on in railroad locomotives until the 1950s, when the major cost savings brought by diesel engines led to its replacement.
2.1 The Four-Stroke IC Gasoline Engine • The four-stroke IC gasoline engine has been the power source for most of the autos and small trucks ever built. • Next slide (Fig. 13.1)A cross-sectional view of a typical auto engine.
Most auto engines have four such piston and cylinders, some have six or eight. • To begin a cycle, with the piston at the top (top dead center, TDC) during the first stroke the piston moves downward while the intake value is open, so that an air-fuel mixture is sucked into the combustion chamber (the space within the cylinder, above the piston).
When the piston is at the bottom (bottom dead center, BDC), the intake valve closes, ending the intake stroke. • As the piston rises again to the top during the compression stroke, both valves are closed, so that the air-fuel mixture is compressed. • Near the top of that stroke the spark plug fires, igniting the air-fuel mixture.
In its next downward travel, the power stroke, the piston is driven by the high-pressure combustion gases, which do the actual work of the engine. • At the bottom of the piston travel, the exhaust valve opens, and on its next upward travel the piston pushes the burned gases out into the exhaust system.
The cycle is named for its four strokes-intake, compression, power, and exhaust. • The spark plug fires every second upward travel of the piston. • Power is produced only during the power stroke. Each of the other three strokes consumes power.
2.2 Pollutant Formation • The principal pollutants emitted from simple gasoline-powered IC engines are carbon monoxide, hydrocarbons, and nitrogen oxides. • Auto engines produce more of them per unit of fuel burned than other combustion processes principally for the following reasons:
Auto engines are often oxygen deficient, which most other combustion systems are not. → CO, HC • Auto engines preheat their air-fuel mixtures, which most combustion systems do not. → NOX
Auto engines have unsteady combustion, in which each flame lasts about 0.0025s.Almost all other combustion systems have steady flames that stand still while the materials burned pass through them. → CO, HC • Auto engines have flames that directly contact cooled surfaces, which is not common in other combustion systems. → HC
2.2.1 Carbon Monoxide, CO • In automobile engines we often have less than stoichiometric air.Let the excess air ratio E be negative values or it is much easier to use the oxygen deficit z. • For any hydrocarbon fuel with formula CxHy: • If there is less than stoichiometric oxygen, we can write:
All gasoline are mixtures of many components, but they can be characterized as having an approximate average formula CxHy, where for a typical gasoline x is about 8 and y is about 17. • Gasoline manufacturers change these values from one location to another and with season of the year (smaller values in winter and in cold climates than in summer and warm climates).
For complete combustion (E = z = 0) of this fuel, the equations is: • If there is not enough oxygen to complete the reaction, then the combustion products will contain CO, H2 and unburned hydrocarbons. • At combustion temperatures the most common of these products of incomplete combustion is CO.
If we assume that an oxygen deficit of z mols per mol of fuel is fed, and if we assume that all of the oxygen deficit causes CO formation, we can rewrite Eq. (1) as: • Each mol of oxygen from air brings with it (0.79/0.21 = 3.76) mols of nitrogen, so that the total mols of combustion products will be:
The mol fraction of CO will be: • Example 1: Calculate the expected CO mol fraction for combustion of a gasoline with x = 8, y = 17, and with the air supplied being 90 percent of that required for complete combustion.
2.2.2 Air-Fuel Ratio (A/F), Equivalence Ratio (Ø) • Example 2: Calculate A/F and Ø for Example 1. • Solution:The A/F is always stated in weight terms in the IC literature (normally lb/lb in the US).In general it is written as:
For this example we have • The equivalence ratio is defined as: • The normalized A/F ratio λ=1/Ø = 0.9.
Table 13.3 (next slide) shows the A/F ratios that actually occur in IC engines.
From Table 13.3, an IC engine would operate satisfactorily for any normalized A/F ratio between 0.5 and 3.5. • However, based on experience with actual engines the operable range is from about 0.8 to 1.3. • The smaller operable range is mostly due to the large heat losses from the small amount of combustible mixture in the cylinder to the surrounding cooled cylinder walls and head.
For steady operation at most driving speeds, the best fuel economy occurs at a λ of about 1.2. • For acceleration or hill climbing the requirement is not best fuel economy but maximum power output, which is found at λ≒0.95. • Most engines idle more smoothly at values between 0.90 and 0.95. (At low speeds there is more time for heat losses to put the flame out.)
Cold starting poses a special problem for IC engines. • When the engine is cold the exhaust heat is not available, and the temperature in the compressed mixture is so low that much of the liquid fuel is not vaporized. • Only the most volatile parts of the fuel will be vaporized under this condition.
To make λ, based on the vaporized part of the fuel, be low enough for the engine to start, one must put more total fuel into the air-fuel mixture. • In carburetor autos, this excess fuel is added by a choke. This was operated by hand on older cars and is now operated by thermostatic or electronic sensors. • Fuel injection engines regulate the amount of fuel injected, taking the same variables into account.
2.2.3 Hydrocarbons (HC) • At all values of λ one measures unburned HC in the exhaust gases of gasoline IC engines. • Most of these are the result of flame quenching.
IC engines must have some kind of lubrication where the piston slides up and down in the cylinder. • In auto engines this is provided by the motor oil, which is pumped from a sump at the bottom of the crankcase through holes drilled or cast in the block, bearings, crankshaft, connecting rods, wrist pins, and cylinders to holes on the side of the piston.
The piston rings, which are the actual sliding surface between piston and cylinder, ride on this oil film. • Normal hydrocarbon lubricants cannot stand temperatures much higher than about 250-3000F (121-1490C) for long periods.
The principal purpose of the cooling system of an auto engine is to keep the temperature of the lubricant film between the piston rings and the cylinder wall at or below the temperature. • Heavily loaded engines, in trucks or autos that pull trailers, have separate radiators to cool the oil.
If the temperature becomes significantly higher than that, the lubricants decompose, leaving behind solid carbon residues that cause the engine to seize. • An engine operated without its cooling system is destroyed in a few minutes.
Research engines have been built that use solid lubricants (MoS2) that can stand very high temperatures. • These engines have no cooling system and operate at temperatures comparable to the melting point of steel. • They have excellent fuel economies but very difficult materials-engineering problems.
The cooling of the cylinder walls and head makes them cold enough that in a narrow quench zone adjacent to them the flame goes out, and the hydrocarbons in that part of the air-fuel mixture are not burned up. • Example 3: Estimate the hydrocarbon concentration to be expected in the exhaust gas from an engine with a piston diameter of 6 cm, a stroke of 5 cm, and a quench zone thickness of 0.2 mm at λ=1.
Solution:We assume that all of the surface of the cylinder and the head has a quench zone.The top of the piston is not cooled and does not apparently play a significant role in flame quenching.
The ratio of the volume of the quench zone to the volume of the combustion chamber with diameter D, piston travel L, and quench zone thickness t (assuming a flat head) is:
Thus we would expect 1.7% of the total hydrocarbons in the fuel to appear in the exhaust. • From Eq.(2), Total mol of combustion products =
mol fraction of unburned fuel in exhaust = yunburned • The calculation shows that we would expect a higher hydrocarbon concentration in the exhaust from a small engine than a large one, which is observed.
This example assumes that the hydrocarbons in the exhaust have the same chemical composition as those in the fuel. • Next slide (Table 13.4)Typical composition of hydrocarbons in untreated auto exhaust.
The methane, ethane, acetylene, propylene, formaldehyde, and other aldehydes were not present in the fuel and must have been formed by incomplete combustion, mostly in the quench zone. • The benzene, toluene, and xylenes were present in the fuel.They are the gosoline components with the slowest burning velocities, and hence the highest probability of passing, unburned, into the exhaust.