Several models exist for measuring the energy consumption of hybrid vehicles.
Several models exist for measuring the energy consumption of hybrid vehicles.
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PHEV Energy consumption of Plug-in Hybrid Electric Vehicles

Author / Editor: Jamie Thomson / Isabell Page

Hybrid vehicles use a combination of fossil fuels and electricity. The electric batteries in Plug-in Hybrid Electric Vehicles (PHEVs) have a high capacity that recharges when the car is stationary. As such, they can offer enough driving range to cover everyday journeys without having to use the combustion engine.

Sales of hybrid vehicles are rising in Europe as environmentally-conscious manufacturers and consumers aim to lower carbon dioxide emissions. However, hybrid vehicles aren’t without their share of bad press. One recent study from Transport and Environment suggests that the newest models pollute the climate more than automotive manufacturers claim. With this in mind, let’s take a closer look at how the energy consumption of a hybrid vehicle is measured:

The benefits of hybrid vehicles

The idea behind hybrid cars is that by combining an electric motor with a petrol engine, they can reduce exhaust emissions. For example, the Honda CR-V Hybrid SUV is claimed to emit just 120g/km of carbon dioxide, which is comparable to what you’d expect from a much smaller vehicle.

Hybrid vehicles also offer better fuel economy. With an electric motor to assist a combustion-powered engine, vehicles can travel further without having to refuel. With a hybrid car, the electric battery doesn’t need to be charged like with a fully-electric vehicle. Instead, when the battery is low, the vehicle switches to petrol and the battery charges itself. The Renault Clio E-Tech, for example, is said to use up to 40 % less petrol when driven in a city as it provides a range of up to 65 kilometers.

What determines the energy consumption of hybrid vehicles?

In order to measure the energy consumption of a PHEV, we need to consider a number of physical and environmental factors. For example, the weight of a vehicle can impact how much energy it uses as lighter vehicles use less fuel. The aerodynamics of a car is also a contributing factor to energy consumption as cars that are more streamlined use less energy.

The nature of the vehicle’s journey needs to be considered as driving uphill, or on winding roads uses more fuel than on a level, straight road. Similarly, driving in a city, where vehicles are constantly stopping and restarting can impact energy consumption.

Likewise, the way a driver handles a vehicle affects how much fuel it uses. For example, driving at high speeds uses more fuel than driving at moderate speeds. Smooth driving with gradual acceleration and deceleration is also more fuel efficient than a more ‘reactive’ driving style.

A recent experiment conducted by Energy Procedia compared the energy consumption of three different models of the same vehicle – the Toyota Yaris Hybrid, the Toyota Yaris 1.5 gasoline and a hybrid Toyota Prius. The results show that energy consumption is influenced not only by the type of vehicle, but also by driving style and speed.

Measuring the energy consumption of PHEVs

There are several methods of measuring the energy consumption of hybrid cars. As one research journal suggests, most energy consumption models have been developed with a specific agenda. Some measure fuel consumption for transportation planning, transportation impact assessments and vehicle technology evaluations. Whereas others measure energy consumption for traffic simulation models and specific control conditions.

While these various models perform their specific purpose effectively, an ideal energy consumption model needs to consider real-world vehicle use. One previous model that aimed to effectively measure the energy consumption of hybrid cars in this way was the EU-funded Surplus Value Hybrid (SUVA) project, which ran from 2001 to 2004. The aim of the SUVA project was to measure energy consumption by considering the engine, transmission and drive axle of a vehicle as well as the state of charge of the battery. Essentially, the model measured the state of battery charge regression.

The current measurement model of energy consumption in hybrid vehicles follows guidelines set by the United Nations Economic Commission for Europe (UNECE). An overview of the exact procedure can be read in a report by the International Council on Clean Transportation. The report sums up how energy consumption should be measured in the future: "The CO2 emission level of a PHEV strongly depends on factors such as the driving distance, consumer behavior, ambient temperature, and recharging behavior…In the future, the specific driving behavior of individual customers could be taken into account for estimating fuel consumption and CO2 emission values that more accurately reflect the everyday driving experience and that allow for a fairer comparison between individual vehicle types."