HEV Fundamentals Hybrid electric vehicles (HEVs) are vehicles that combine an internal combustion engine (ICE) with an electrical traction system. It usually consists of either two or more sources of energy storage devices or two or more power sources onboard the vehicle. HEVs are vehicles that offer high fuel economy and lower emissions when compared to conventional gasoline vehicles.

Hybrid vehicles combine the ICE and electrical traction machine in an efficient way so as to utilize the most desirable characteristics of both. In HEVs, the ICE is mainly used for steady state operation while the electric machine powertrain is mainly used for dynamic operation. Some of the advantages offered by HEVs are as follows:

Efficiency-improving technology such as regenerative braking which is not available in conventional vehicles. Less engine idling and efficient engine operation leading to better fuel economy. Better drivability since electric motor characteristics better match the road load. Potential to reduce the emission of greenhouse gases. Reduced fossil fuel consumption.

HEVs can be classified based on the configuration of the drivetrain as series hybrid, parallel hybrid, series–parallel hybrid, complex hybrid, and plug-in hybrid.

In general, when we design a HEV, we need to select the ratings for the propulsion engine, traction electric motor, generator, and energy storage based on the desired vehicle performance. After the initial design, we need to verify if the vehicle performance specifications are met. The design usually requires a modeling and simulation program and may take several iterations before final design.

Vehicle Model In this section, the vehicle is modeled as a road load. The vehicle and the associated forces are illustrated in the following Figure.

The force due to the road grade depends on the mass of the vehicle Mv, road angle in degrees α, and gravitational acceleration g. The equation for this force is:

The road load curves of a vehicle for varying road angles are shown in the following Figure. The vehicle parameters are listed in the following table Table. It can be observed that the road load increases with the velocity and with road angle.

Vehicle Performance For any vehicle design, the performance constraints to be met must be defined first. These constraints are different depending on the vehicle type and size. From the powertrain point of view, typical performance specifications include initial acceleration, cruise speed, maximum speed, gradability, drive range, and so on. Acceleration rate is the minimum time required to accelerate the vehicle from 0 to a specified speed such as 40, 60, or 80 mph. Sometimes acceleration rate from a lower speed to a higher speed is specified; for example, from 40 to 60 mph.

Maximum acceleration is limited by maximum tractive power and roadway condition. The gradability is the maximum grade that a vehicle can move along at a certain speed with the maximum tractive force available from the powertrain. Drive range refers to the distance, in miles or kilometers, that a vehicle can travel with a full tank of fuel and/or fully charged batteries before refueling or recharging. Satisfactory drive range of an electric vehicle (EV) or HEV is crucial for market acceptance.

The US PNGV’s (Partnership for a New Generation of Vehicle’s) performance goal for mid-size vehicles is as follows: 0–60 mph: ≤12 seconds 40–60mph: ≤5.3 seconds 0–85 mph: ≤23.4 seconds Maximum speed: 85 mph Maximum grade at 55 mph: 6.5%.

Drive cycles are standard vehicle speed versus time profile for testing vehicle performance, fuel economy, and emissions. For example, the Federal Highway Driving Schedule (FHDS) and Federal Urban Driving Schedule (FUDS) are plotted in the following Figure:

The required power for operating a vehicle can be calculated from the driving cycles, depending on the mass of the vehicle. For example, the required power for driving a vehicle with a total weight of 1380 kg under US06 driving cycle is shown in the follwoing Figure. US06, the aggressive driving cycle, was proposed by the US Environmental Protection Agency (EPA) to measure fuel economy and emissions. The positive power is due to acceleration while the negative power is due to deceleration. Part of the negative power corresponding to braking can be recovered through regeneration in the HEV.

The fuel economy refers to how many miles a vehicle can travel with the consumption of per unit of fuel. One common unit is miles per gallon or MPG. For EVs or HEVs, miles per gallon gasoline equivalent (MPGGE) is used to measure how many miles a vehicle can travel with the consumption of energy equivalent to the amount released from combustion of 1 gallon of gasoline. Fuel economy of HEVs also depends on the drive cycles. Thus, sometimes, composite fuel economy or combined fuel economy is used. For example, composite fuel economy can be computed as the weighted average of the state of charge (SOC) balanced fuel economy values during the city drive cycle and highway drive cycle, as given below:

EV Powertrain Component Sizing As discussed earlier, when we size the powertrain of an EV, we must ensure sufficient tractive force for the vehicle to: accelerate from zero speed to a certain speed within a required time limit; overcome wind resistance force if headwind speed is non-zero; overcome aerodynamic force; overcome rolling resistance; climb a certain slope (grade).

The power and energy requirement from the powertrain is determined from a given set of vehicle cruising and acceleration specifications. EV/HEV design is an iterative process and requires many engineers from multiple disciplines to collaborate to meet design goals. Thus:

Electrical and mechanical engineers design the electric motor for the EV or the combination of electric motor and ICE for HEVs. Power electronics engineers design the power conversion circuit which links the energy source with the electric motor. Control engineers working in conjunction with the power electronics engineers develop the propulsion control system. Electrochemists and chemical engineers design the energy source based on the energy requirement and guidelines of the vehicle manufacturer.