The Prius Prime combats cold-weather range anxiety with a high-tech solution for cabin heat.
Hybrid, plug-in hybrid and pure electric cars are being designed by every major automotive company in the world. The cars can be designed for performance and/or economy, but as with any new technology, there are always some things to work out.
In the case of vehicles, it is the heater.
Heat is no problem with internal combustion engines. Only a small percentage of the fuel energy is used to move the vehicle. The rest of the energy is converted to heat and expelled through the radiator and the exhaust for our comfort in the heater system. Hybrid vehicles can also use the internal combustion engine as a source of passenger compartment heat, but that is only possible when the engine is running. When operating in electric mode, there is no source of heat, so if the passengers require heat, the engine has to start — which reduces economy.
Electric vehicles have a problem. Their only source of heat is electric heating elements or an electrically driven heat pump, which consume power from the vehicle battery. This consumes a lot of energy and can reduce a vehicle’s operating range by up to 60 per cent. This not only reduces the capability of the vehicle, but can cause “range anxiety,” where the driver is constantly concerned about being able to drive the distance because the battery power is being quickly consumed.
Now there is something new that can help — Toyota’s gas-injection, heat-pump system.
The gas-injection, heat-pump system has nothing to do with fuel injection or gasoline. The system name refers to the gas vapour contained in the vehicle’s air-conditioning system and the injection is a new way of using that vapour. The system is built into Toyota’s 2017 Prius Prime, a plug-in hybrid vehicle, but the system has the capability of being adapted to any vehicle on the road.
The 2017 Prius Prime uses a heat pump air-conditioning/heating system. Heat pumps are not new. A basic heat-pump system is simply an air-conditioning system operating in reverse, so the cold air is rejected outside the passenger compartment and warm air is delivered inside the passenger compartment. Unfortunately, heat pumps do not work well when temperatures get really cold, so the Prius Prime system works only to -10 C temperatures, which may not seem that cold, but most of the country has winter temperatures above that range for a large percentage of the time.
The innovation in the Prius Prime system is the incorporation of a cyclone separator (CS) integrated valve into the system. To understand how this works, an understanding of conventional heat pump operation will help. In a normal heat-pump system, the compressor pumps high-pressure refrigerant vapour to the evaporator core inside the passenger compartment. The compressed vapour is hot and transfers the heat to air blowing through the heater system. This cools the vapour and it starts to turn into a liquid, which flows under high pressure out to the control valve where flow is restricted and the high-pressure liquid turns into a low-pressure liquid that starts to boil. It continues to boil as it passes through the condenser in front of the radiator, absorbing heat from the outside air until the refrigerant is a vapour again and it goes back into the compressor to go through the cycle again. A heat pump is the opposite of the flow when air conditioning is requested.
Now, Toyota has added the Cyclone Separator integrated valve into the system, which uses centrifugal force inside the valve to separate the vapour from any liquid and injects the vapour directly back to the compressor, bypassing the condenser. The vapour is already warm, so it can provide more heat to the vehicle interior, while the liquid is directed back to the condenser, where it can absorb more heat from outside the vehicle. There are now two paths for the vapour/liquid to flow and the efficiency of the system is greatly increased.
Toyota claims the system will use 63 per cent less energy than a traditional heating system and will increase the range of the vehicle on electric power by 21 per cent in cold weather.
For very cold weather, heat can still be obtained from the internal combustion engine, but there are six different modes of heating operation when the vehicle is operating in electric mode to maximize economy.
Toyota has taken a simple principle and made it technically feasible through innovation to make vehicles better.