The B2B platform for full-electric and plug-in hybrid electric vehicles: Industry News

Braking and air-conditioning systems tailored to electric vehicles, are this week's subjects of our overview of Japanese media reporting on developments in the electric vehicle market. We take a closer look at the braking systems of the iMiEV and the HEV Prius as well as the iMiEV's climate control system.

It does not suffice to replace the engine with an electric motor to build an EV. All systems of the car need to be adjusted in order to perform well in an electrically driven vehicle, especially core systems such as the breaking - key to the car's safety, and air-conditioning - key to the passengers' comfort.

Special braking system for electric vehicles

Virtually all of today's conventional cars have brake boosters to multiply the force that the driver's foot applies via the brake pedal on the master cylinder to slow down the car. This brake boosters use the vacuum created when the engine sucks in air and gasoline. Electrically powered cars, however, do not dispose of this assist energy for braking. They also lack the important source of emergency braking by engine compression - braking through down-shifting.

The braking effect of downshifting is somewhat recreated in regenerative braking systems that use the braking energy to recharge the batteries. Mitsubishi's full electric iMiEV, for example, will be equipped with a regenerative braking system plus an electric vacuum pump that feeds the brake booster, resulting in braking performance similar to conventional cars.

The hybrid vehicle brake system of the HEV Toyota Prius on the other hand uses a regenerative brake cooperative control system, including both standard hydraulic brakes and a regenerative braking system. As soon as the accelerator pedal is released, the high voltage electronic control unit (HV ECU) initiates regenerative braking. One of the two electric motors is turned by the wheels and used as a generator to recharge the batteries. When more rapid deceleration is required, the hydraulic brakes are activated to provide additional stopping power.

In a conventional car, the force that the driver's foot is exercising on the pedal is transmitted and multiplied via fluid compression to the brakes. In the Prius, however, the pedal becomes like an on/off switch. But the degree of pressure put on the pedal by the driver is still important as sensors are measuring the force and speed deployed and use these measures to calculate the necessary braking power of each wheel.

Special air-con system for electric vehicles

Air-conditioning systems as well will have to be adapted for electric vehicles. In conventional cars, waste heat from the combustion engine is through the coolant transferred to the heat exchanger, ambient air is heated and pumped into the passenger compartment. Or the engine drive is used to power a compressor unit, which compresses the coolant that travels further to the condenser. The condenser puts the gas in contact with fresh air on the outside of the vehicle, which absorbs the heat from the gas. This causes the refrigerant to liquefy. It then flows into the dryer, where the liquid is purified. The clean liquid then travels through the tubing into the evaporator, where it turns into vapor. Fans blow the air from the car's cabin over the evaporator, and the evaporating liquid sucks the heat out of the air to use as energy for the evaporation process. The cool, dry air is recirculated back into the cabin.

However, an electric vehicle has neither the waste heat from the engine nor the engine's drive force. Mitsubishi has therefore developed a climate control system for the iMiEV that combines air-conditioning with an electrically powered compressor and a heating system which circulates warm water heated by an electric heater. The electric compressor only runs when needed and therefore achieves higher efficiency than a compressor driven by engine power. However, both air-conditioning and heating are energy intensive and therefore accelerate battery depletion.


The original Japanese article is listed below.