hybrid cars

Background

Internal combustion engines are the major contributors to the air pollution in California.

Reducing vehicular emissions and enhancing fuel economy will be effective in improving

the air quality. Recent advances in diesel combustion technologies, better afterburners

and catalytic oxidizers, and use of alternative fuels such as compressed natural gas

(CNG), propane, and methanol have resulted in overall reduction in the emission of

particulate matters (PM) and other gaseous emissions such as volatile organic

compounds, nitric oxides, and oxides of carbons. In light of increased use of cars and

other public transportation, and ever more congested traffics, additional steps must be

taken to reduce emissions even further. The advantages and disadvantages of electric

vehicles are generally known and accepted. Electric vehicles help the environment by

eliminating exhaust emissions and reducing dependency on fossil fuels. However, the

disadvantages of limited range and increased vehicle weight limit their use in commercial

applications. Hybrid-electric vehicles solve many of the problems plaguing pure electric

vehicles such as short range and excessive weight, battery cost and battery life.

Commercial hybrids have been in production in Japan and are soon to be introduced in

US Market. The Toyota Prius has demonstrated the superiority of these vehicles by

getting 60 MPH in city driving and 70 mph in highway driving while at the same time

producing emission at one tenth of the legal limits. Honda_s Insight with similar

performance is scheduled for release soon.

Because series hybrids are simpler and thus are of a lesser investment risk, they

have been generally favored by the manufacturers. Parallel hybrids however, are

expected to offer the best fuel economy because they can recover energy from

regenerative braking and can directly use the energy from the fuel converter without the

need to convert is first to electricity. No commercial parallel hybrid vehicle is in

production, but many manufacturers are investigating their merits.

Unlike the conventional vehicles where engines are directly coupled to drive trains,

hybrid vehicles speed is determined by a number of control parameters not directly

related to the engine loads. For example, in conventional vehicles, accelerator pedal

(load) directly determines the engine speed, and the rate of fuel delivery and gas mileage.

Hybrids on the other hand, operate on the principle that the total power delivered by the engine and the battery must be sufficient to satisfy the load requirement while

maintaining battery charge power. The power drained out of the engine thus can be

changed depending not only on the required motive power, but also to the state of the

charge of the batteries.

Compared to conventional buses, hybrid buses offer considerably less emissions.

Most of the pollution from a conventional diesel bus is a result of transients, and vary

as the power delivered by the engine to the drive axle (generally rear axles). With

hybrids, the power delivered by the engine may or may not follow vehicle speed and

load. In certain configurations, the engine could be much smaller and even run at a

steady speed, thus cutting emissions significantly. Although manufacturers of hybrid

vehicles design the system to operate in an optimum performance range, (reduced

emission and better fuel economy), situations can be visualized where either one

criteria or other are of primary importance.

Whether series or hybrid configurations are considered, these vehicles cannot be

considered optimized for all drive cycles and for all applications. Control strategies are

often a simple on/off switch that instructs the vehicle to operate as electric or gasoline

vehicle. Thresholds are fixed by the manufacturers irrespective of criteria of interest.

For example, a vehicle that might give a superior gas mileage for a given drive cycle

might be quite ineffective in reducing emissions and for a different driving cycle. The

air quality problem is of a much greater concern than fuel economy in many of the

cities around the world.

The long-term goal of the project is to obtain a set of control parameters that can be used

to optimize the operation of vehicles (fuel efficiency, pollution, or both) depending on a

particular drive cycle (route), and strategy (minimize pollution or maximize fuel

economy). Once such variables are found and each vehicle is tuned for optimal operation,

then better routing and scheduling can increase throughput over a given metropolitan

district, and by doing so, expand services to remote locations without increasing the

overall cost.