Other Automotive Power Plants

.In an attempt to reduce fuel consumption and harmful exhaust emissions, many manufacturers are supplementing or modifying the basic internal combustion engine. Many of these power plants were developed during the early days of automobiles. Due to the advancements made in electronic controls, they are becoming a viable alternative to the conventional gasoline engine.

+++ The configuration of a parallel hybrid vehicle. Combustion engine Electric motor/generator Fuel tank Electrical

storage Generator

+++ The configuration of a series hybrid vehicle.

Combustion

engine Electric motor/ generator Fuel tank Electrical storage

+++ The sources of power for a fuel cell electric vehicle: fuel cell stack (left), power control unit (center), and lithium ion battery pack (right).

Hybrids

A hybrid vehicle has at least two different types of power or propulsion systems. Today's hybrid vehicles have an internal combustion engine and an electric motor (some vehicles have more than one electric motor). A hybrid's electric motor is powered by batteries and/or ultracapacitors, which are recharged by a generator that is driven by the engine. They are also recharged through regenerative braking. The engine may use gasoline, diesel, or an alternative fuel. Complex electronic controls monitor the operation of the vehicle. Based on the current operating conditions, electronics control the engine, electric motor, and generator.

Depending on the design of the hybrid vehicle, the engine may power the vehicle, assist the electric motor while it’s propelling the vehicle, or drive a generator to charge the vehicle's batteries. The electric motor may propel the vehicle by itself, assist the engine while it’s propelling the vehicle, or act as a generator to charge the batteries. Many hybrids rely exclusively on the electric motor(s) during slow speed operation, on the engine at higher speeds, and on both during some certain driving conditions.

Often hybrids are categorized as series or parallel designs. In a series hybrid, the engine never directly powers the vehicle. Rather it drives a generator, and the generator either charges the batteries or directly powers the electric motor that drives the wheels. Currently there are no true series hybrids manufactured. A parallel hybrid vehicle uses either the electric motor or the gas engine to propel the vehicle, or both. Most current hybrids can be considered as having a series/parallel configuration because they have the features of both designs.

Although most current hybrids are focused on fuel economy, the same construction is used to create high-performance vehicles. The added power of the electric motor boosts the performance levels pro vided by the engine. Hybrid technology also enhances off-the-road performance. By using individual motors at the front and rear drive axles, additional power can be applied to certain drive wheels when needed.

The engines used in hybrids are specially designed for fuel economy and low emissions. The engines tend to be small displacement engines that use variable valve timing and the Atkinson cycle to provide low fuel consumption. These advanced engines, however, cannot produce the power needed for reasonable acceleration by themselves.

The electric motor provides additional power for acceleration and for overcoming loads.

Battery-Operated Electric Vehicles--A battery operated electric vehicle, sometimes referred to as an EV, uses one or more electric motors to turn its drive wheels. The electricity for the motors is stored in batteries that must be recharged by an external electrical power source. Normally they are recharged by plugging them into an outlet at home or other locations.

The recharging time varies with the type of charger, the size and type of battery, and other factors. Normal recharge time is 4 to 8 hours.

An electric motor is quiet and has few moving parts. It starts well in the cold, is simple to maintain, and does not burn petroleum products to run. The disadvantages of an EV are limited speed, power, and range as well as the need for heavy, costly batteries.

However, an EV is much more efficient than a conventional gasoline-fueled vehicle. EVs are considered zero emissions vehicles because they don’t directly pollute the air. The only pollution associated with them is the result of creating the electricity to charge their batteries.

In the early days of the automobile, electric cars outnumbered gasoline cars. Today, there are few EVs on the road but they are commonly used in manufacturing, shipping, and other industrial plants, where the exhaust of an internal combustion engine could cause illness or discomfort to the workers in the area.

They are also used on golf courses, where the quiet operation adds to the relaxing atmosphere. Some auto manufacturers are still studying their use.

Whether battery-operated EVs return to the market really depends on the development of new batteries and motors. To be practical, EVs need to have much longer driving ranges between recharges and must be able to sustain highway speeds for great distances.

Fuel Cell Electric Vehicles--Although just experimental at this time, there is much promise for fuel cell EVs. These vehicles are powered solely by electric motors, but the energy for the motors is produced by fuel cells. Fuel cells rely on hydrogen to produce the electricity. A fuel cell generates electrical power through a chemical reaction. A fuel cell EV uses the electricity produced by the fuel cell to power motors that drive the vehicle's wheels. The batteries in these vehicles don’t need to be charged by an external source.

Fuel cells convert chemical energy to electrical energy by combining hydrogen with oxygen. The hydrogen can be supplied directly as pure hydrogen gas or through a "fuel reformer" that pulls hydrogen from hydrocarbon fuels such as methanol, natural gas, or gasoline. Simply put, a fuel cell is comprised of two electrodes (the anode and the cathode) located on either side of an electrolyte. As the hydro gen enters the fuel cell, the hydrogen atoms give up electrons at the anode and become hydrogen ions in the electrolyte. The electrons that were released at the anode move through an external circuit to the cathode. As the electrons move toward the cathode, they can be diverted and used to power the vehicle.

When the electrons and hydrogen ions combine with oxygen molecules at the cathode, water and heat are formed. There are no smog-producing or greenhouse gases produced. Although vehicles equipped with reformers emit some pollutants, those that run on pure hydrogen are true zero-emission vehicles.

Rotary Engines:

The rotary engine, or Wankel engine, is somewhat similar to the standard piston engine in that it’s a spark ignition, internal combustion engine. Its design, however, is quite different. For one thing, the rotary engine uses a rotating motion rather than a reciprocating motion. In addition, it uses ports rather than valves for controlling the intake of the air-fuel mixture and the exhaust of the combusted charge.

The main part of a rotary engine is a roughly triangular rotor that rotates within an oval-shaped housing. The rotor has three convex faces and each face has a recess in it. These recesses increase the overall displacement of the engine. The tips of the rotor are always in contact with the walls of the housing as the rotor moves to seal the sides (chambers) to the walls.

As the rotor rotates, it creates three separate chambers of gas. Also, as it rotates, the volume between the sides of the rotor and the housing continuously changes. During rotor rotation, the volume of the gas in each chamber alternately expands and contracts. It’s how a rotary engine rotates through the basic four stroke cycle.

The rotor "walks" around a rigidly mounted gear in the housing. The rotor is connected to the crank shaft through additional gears that allow every rotation of the rotor to rotate the crankshaft three times.

This means that the output shaft only rotates three times for every revolution of the rotor, which allows only one power stroke for each revolution of the out put shaft. This is why a rotary engine produces less power than a conventional four-stroke engine. When more than one rotor is fitted inside the engine, each rotor is out of phase with the others and the power output is increased.

Referring to the diag., when the side of the rotor is in position "A," the intake port is uncovered and the air-fuel mixture is entering the upper chamber. As the rotor moves to "B," the intake port closes and the upper chamber reaches its maxi mum volume. When full compression has reached "C," the two spark plugs fire, one after the other, to start the power stroke. At "D," the side of the rotor uncovers the exhaust port and exhaust begins. This cycle continues until the rotor returns to "A" and the intake cycle starts once again.

The rotating combustion chamber engine is small and light for the amount of power it produces, which makes it attractive for use in automobiles. However, the rotary engine at present cannot compete with a piston gasoline engine in terms of durability, exhaust emissions, and economy. After a few years of not offering a rotary engine, Mazda has released a version of the engine, called the Renesis, that produces lower emissions and has two rotors.

+++ A rotary engine cycle.

+++ A typical stratified charge engine.

Stratified Charge Engines:

The stratified charge engine combines the features of gasoline and diesel engines. It differs from the conventional gasoline engine in that the air-fuel mixture is deliberately stratified to produce a small rich mixture at the spark plug while providing a leaner, more efficient and cleaner burning main mixture. In addition, the air-fuel mixture is swirled to provide for more complete combustion.

A large amount of very lean mixture is drawn through the main intake valve on the intake stroke to the main combustion chamber. At the same time, a small amount of rich mixture is drawn through the auxiliary intake valve into the precombustion chamber. At the end of the compression stroke, the spark plug fires the rich mixture in the precombustion chamber. As the rich mixture ignites, it in turn ignites the lean mixture in the main chamber. The lean mixture minimizes the formation of carbon monoxide during the power stroke. In addition, the peak temperature stays low enough to minimize the formation of NOx, and the mean temperature is held high enough and long enough to reduce hydrocarbon emissions.

The Honda CVCC engine uses a stratified charge design. This engine uses a third valve to release the initial charge. The stratified charge combustion chamber has three important advantages: It produces good part-load fuel economy, it can run efficiently on low-octane fuel, and it has low exhaust emissions.

Homogeneous Charge Compression Ignition Engines:

Within the next few years, some automobiles will be equipped with homogeneous charge compression ignition (HCCI) engines. HCCI engines offer the high efficiency and torque of a diesel engine while providing the low emissions and power of a gasoline engine.

Basically these engines have a combustion process that allows a gasoline or diesel engine to operate with either compression ignition or spark ignition. With spark ignition the air and fuel are mixed (homogenized) before ignition and ignition is caused by a spark. In a diesel engine the air and fuel are never mixed. The air is compressed and ignition occurs when fuel is sprayed into the high-temperature air.

In an HCCI engine, the air and fuel are mixed and ignition occurs as the mixture is compressed.

During compression, the mixture gets hot enough to "autoignite." HCCI is also referred to as controlled auto-ignition (CAI).

In an HCCI engine, combustion immediately and simultaneously begins at several points within the mixture. This means the combustion process occurs rapidly and is controlled by the quality and temperature of the compressed mixture. This spontaneous combustion produces a flameless release of energy to drive the piston down.

The HCCI engine runs on a lean, diluted mixture of fuel, air, and exhaust gases. Only the heat inside the cylinder determines when ignition will occur.

This fact makes it hard to control ignition timing. The temperature of the mixture at the beginning of the compression stroke must be increased to autoignition temperatures at the end of the compression stroke. Autoignition usually occurs when the temperature reaches 1,430°F to 1,520°F (777°C to 827°C) for gasoline. The engine's control unit must supply the correct amount of fuel mixed with the correct amount of air in order for combustion to occur at the right time. In addition, the control unit must provide a mixture that is hot enough to be able to autoignite at the end of the compression stroke. Therefore, it must be able to vary the compression ratio, the tempera ture of the intake air, the pressure of the intake air, or the amount of retained or re-inducted exhaust gas.

The role of the control unit is extremely important for proper operation.

Dual Mode A practical application of an HCCI engine would be one with dual mode capabilities. The spark ignition mode could be used when high power is required, and the compression ignition mode would be used during steady loads and speeds. To do this, the engine must be able to smoothly switch from the HCCI mode to the spark ignition mode from one cylinder firing to the next. This would require precise control of valve timing, air and fuel metering, and spark plug timing.

Benefits--A gasoline HCCI engine could deliver almost the same fuel economy as a diesel engine and at a much lower cost. GM estimates that HCCI could improve gasoline engine fuel efficiency by 20%, while emitting near-zero amounts of NOx and particulate matter. In fact, HCCI engines emit extremely low levels of NOx without a catalytic converter.

However, a gasoline engine running in the HCCI mode produces more noise and vibrations than a conventional engine. Also, they tend to experience incomplete combustion, which leads to hydrocarbon and carbon monoxide emissions. To rectify this, HCCI engines are fitted with typical emission control systems, including an oxidizing catalytic converter.

Variable Compression Ratio Engines:

Variable compression engines are being explored, not only for use with HCCI, but for use in conventional engines. Changing the compression ratio is one way to provide power when needed and minimizing fuel consumption. One way to do this is through changes in valve timing. The process is similar to the modifications made for the Atkinson cycle. Another way is to change the volume of the combustion chamber in response to the engine's operating conditions.

Saab has developed such an engine, called Saab variable compression (SVC), that has a cylinder head constructed with integrated cylinders. The compression ratio is altered by changing the slope of the cylinder head in relation to the engine block. This changes the volume of the combustion chamber. The cylinder head is pivoted at the crankshaft by a hydraulic actuator and can be as much as 4 degrees. The engine management system adjusts the angle in response to engine speed, engine load, and fuel quality. The cylinder head is sealed to the engine block by a rubber bellows.

+++The SVC can vary the engine's compression ratio from 8:1 to 14:1.

Next: Engine Identification

Prev.: Diesel Engines