Poulsen Hybrid, Inc.

Plug-In Hybrids

 

Background


2007    USA: Gasoline $3.25 per Gallon.       Electricity  $0.15 per Kilowatt hour
             Europe: Gasoline $7.00 per Gallon.  Electricity   $0.20 per Kilowatt hour
 
The efficiency of gasoline engines in traditional cars is around 20%. Only one fifth of the energy content in the fuel is utilized for propulsion.

One gallon of gasoline equals about 35 KWh of energy.

80% of gasoline used in a traditional car, or 28KWh per gallon, is wasted in the form of heat and only 20% or 7KWh turned into mechanical work. In a light car getting 35 miles per gallon this amounts to 200 watt-hours per mile. If it were possible to power a car directly from a wall outlet and with electricity priced at 15 cents per kWh, the cost of 200 watt-hours is 3 cents. However, since the total efficiency of charging a bank of batteries and using the energy to drive a vehicle is only around 75 %, the actual cost of 200 watt-hours for use in an electric vehicle amounts to 4 cents.

US Cost per mile of transportation to the user and to the environment:

  Fuel CO2 released
 Full size car, 20 miles/gallon 16 cents  1.1 lb
 Compact car, 35 miles/gallon 9 cents 0.62 lb
 Gasoline hybrid car, 45 miles/gallon 7 cents  0.55 lb
 Electric vehicle charged from the grid 4 cents  0.35 lb


However, since most charging takes place at night, often at a reduced rate, the actual cost of running an all-electric vehicle may be lower depending on location. Also, if renewable energy is available for charging, the CO2 emission is zero.

Lower cost per mile makes a strong case for electric vehicles, only marred by the fact that battery technology hasn't quite caught up with expectations. Even with today's most advanced batteries it is not economically viable to design an electric vehicle with a range much above 100 miles, or less than a third of the range of most conventional cars. The risk of running empty out of reach of charging facilities represents a serious problem to most car owners. Even with access to a power outlet charging takes several hours. Batteries cannot be charged nearly as fast as filling a gas tank.

 

Hybrid Vehicles

A Hybrid Electric Vehicle (HEV) contains an internal combustion engine and an electromotor/generator plus a bank of batteries. The role of the electromotor is to enable the gasoline engine to run at its most economical rpm at all times, and when pulling together the two power plants provide a quite impressive acceleration at a considerably lower fuel consumption than that of a comparable traditional car. A HEV is self-contained and not intended to be charged from a power outlet.

 

Plug-In Hybrid Electric Vehicles, PHEV, contain electric motors powered by batteries, which are charged from the electric grid, plus an internal combustion engine for taking over when the batteries are running low. 

 

An obvious way to create a PHEV is to start with a regular hybrid, HEV, which already features both a gasoline engine and an electric motor. Such conversions, mostly of Toyota Prius, are now available from several companies in the US and Canada, who are adding battery capacity and modifying the control systems, enabling vehicles to reach 75-100 miles per gallon during the first 20-30 miles after charging. HEVs are complicated, relatively heavy and cost 20-30% more than comparable conventional automobiles, and even though the metamorphosis to a PHEV raises the ante by $5-8000, many individual and fleet conversions are now taking place, often motivated by environmental concerns. The program has not been wholeheartedly embraced by Toyota, probably because converting a regular HEV is not an ideal proposition to begin with. The reason why this so is that each of the two categories of hybrid vehicles requires a different set of means to reach its specific goal.

Experience indicates that an efficient HEV requires an amount of electric power of largely the same order as the power of the combustion engine. The Prius drive train for example features an engine rated at 77 horsepower paired with a 68 horsepower electric motor/generator and a short-term storage battery. The role of the motor is to enable the prime mover, the gasoline engine, to run at it's most economical rpm at all times, and when pulling together the two power plants provide a quite impressive acceleration while still maintaining a very respectable gas mileage.

A Plug-In Hybrid, PHEV, on the other hand requires a much larger battery in order to reach an all-electric range of 20-30 miles per charge. At a consumption of about 150 watt-hours per mile this range requires a battery capacity of 3.5 to 5 kilowatt hours, only sufficient to operate the 68 horsepower electromotor in the Prius at capacity for 4 to 6 minutes. At the same time the gas engine in a PHEV must be adequate to cope with short-term loads such as acceleration and climbing hills on its own after the battery power has been depleted. 

In this context it is also relevant that a traditional car getting e.g. 30 miles per gallon utilizes only 20 horsepower on the average and closer to 10 HP while running at a steady 60 mph on a level road.  A battery bank with for example 4 kilowatt hours of useable energy can support 10 HP in 33 minutes and 20 HP in only 16 minutes, leading to the conclusion that an electric motor any larger than 20HP is overkill. In the case of a PHEV it would be redundant to have two power systems both able to deal with short-term peak loads. Acceleration and hill-climbing can be handled by a power train like that of a sensibly designed conventional car, and the ideal PHEV can then be created by adding a relatively low power electric motor to serve independently or in "blended mode" adding torque and saving fuel during the daily commute.