Fuel Economy for electric vehicles and hybrids

This blog post discusses integration of hybrids and electric vehicles in fuel economy systems with challenges and proposed solutions. We also discuss why a fuel economy system is motivated in those types of vehicles.

The conversion from combustion engine vehicles to hybrids and electric vehicles presents a challenge for fuel economy measuring systems, because not all parameters used in traditional fuel economy measuring are relevant for hybrids or electric vehicles. Developing parameters and how to examine the results is an ongoing process, and since measuring electric vehicles and hybrids is a relatively new undertaking, adjustments will probably be quite frequent in the near future. We will however present some basic rules and thoughts on how to address the issues at hand.

General idea of fuel economy

What is the purpose and general idea behind a fuel economy system? Usually the main function is based on presenting information to the drivers to enable them to change driving behavior based on the presented information. The changed driving behavior in turn leads to reduced fuel consumption, wear and tear of the vehicle and reduction of the carbon footprint.

Hybrid vehicles

The first thing to note when implementing fuel economy support for hybrids is that “hybrid” is a common name for different techniques. Typically, there are two different types of hybrid solutions with their own  technical solution. The two types are Parallel Hybrids and Serial Hybrids.

Difference between parallel and serial hybrid.

In the case with parallel hybrids, both the electric engine and the combustion engine can drive the wheels, whereas in the case with serial hybrids, only the electric engine is directly connected to the propulsion. The parallel hybrid operates with a smaller electric engine and a smaller battery. The serial hybrid on the other hand, doesn’t need as big a combustion engine.

Regenerative brake

Both types can have a regenerative brake, which means that the braking system at least partly consists of a generator that charges the battery when braking. In the case of parallel hybrids, it is often the electric engine itself that has the possibility to work as a generator.

Parallel Hybrids

In the case of parallel hybrids, the vehicle optimally accelerates to a speed of X with the help of the electric engine. After the acceleration is completed, the combustion engine takes over and is used until the vehicle stands still the next time. If the acceleration is too powerful, the shift over to the combustion engine occurs earlier, with increased fuel consumption as a result.

More complex

The parallel hybrid is more complex than the serial to its nature. Compared to a conventional vehicle it is first and foremost the ability to shut the combustion engine down during idling and using the electric engine to accelerate up to a certain speed, coupled with a regenerative brake with reuse of energy that provides the reduction in fuel consumption.

Serial Hybrids

Serial hybrids are driven solely by the electric engine, which is fed electric power via a super capacitor or directly from the generator. The super capacitor/battery is charged either by the combustion engine generator or through regenerating energy from momentum at retardation. Under some circumstances the electric engine demands direct power from the generator.

Serial hybrid vs. conventional vehicle

The serial hybrid has the benefit compared to a conventional vehicle that the combustion engine can work at an optimal speed and doesn’t have to run during idling. There are however losses when the energy goes through an extra conversion before actually being used to drive the wheels.

Different optimal field of operation

Examinations of serial- and parallel Hybrids show differences in fuel economy depending on the route. Parallel Hybrids perform better at high speeds (>50 km/h) and at very low speeds (<10 km/h). In situations with many stops and accelerations, the serial hybrids perform better. This is due to that the electric engine works more effectively over a broader spectrum of speeds than a combustion engine. This opens for using the eco-driving system for measuring routes to suggest which type of bus that should traffic them.

Fuel economy focus strategy

To lower the fuel consumption in a consistent way, it is wise to focus on driving behavior. This way the difference between vehicles and other, outer circumstances are reduced, and a more fair and predictable result for the users is possible to obtain. Today, a common way to measure driver performance, is to base the results or grades on the driver’s performance in a few categories, like brake, idling and speed, because they all have a significant impact on the fuel consumption for a combustion engine vehicle.

New eco-driving measuring model

For electric vehicles and hybrids, a new incentive model has to be constructed. The new model must be perceived as fair by the drivers. Therefore it can’t be easier to achieve good results in some vehicles than in others. For electric vehicles and hybrids, where idling in reality hardly exists, an incentive in that category would be unfair and useless. If you on the other hand just take away an incentive category for electric vehicles and hybrids, it will be harder for drivers of these vehicles to achieve a good grade.

Brake model adjusted

The brake category can be used in electric vehicles and hybrids as in other vehicles, but with a slightly different approach. Because of the energy regeneration, the dynamic consumption generated by the brake in an electric and hybrid vehicle is less. Therefore the braking for electric vehicles and hybrids has to be graded differently, with a reduction in the braking score.

The driver who to a great extent uses the regenerating brake for retardation should easier achieve a good grade since the dynamic consumption is considerably less for this type of brake. It would also be wise to display the regeneration brake differently in the driver console, to visualise the positive effect of this type of brake, compared to the normal brake where all the energy is lost.

Speeding model works fine

The speeding category can still be used as is for electric vehicles and hybrids. The energy loss that occur due to extra drag, when driving in higher speeds, is the same on electric buses as to other buses.

Coasting not recommended

This category is unclear – it is not obvious if braking or coasting is better when driving, due to that regeneration occurs and has impact on the Fuel Economy calculation. It is not clear if the lost momentum due to coasting is lower than the lost energy from braking with regenerating brakes. There is also a wide variety between brands and models.

Coasting is therefore not recommended as a category to put too much weight on initially. The base for grade calculation of coasting is the number of meters that the driver has coasted divided by the total amount of meters driven. The result of this is a percentage.

Comfort model works fine

The comfort category can be used as is for electric vehicles and hybrids.

Regeneration, a new category

The new category, regeneration, measures the driver performance. The raw regeneration value is measured against the total energy consumed. Compared to how many watts have been consumed. E.g. 2W regenerated, 6 W consumed. 2/6 = 33%. This value can exceed 100% if only downhill, while it will result in a bad grade uphill, so underlying calculations take such factors into account.

This comparison was chosen because it is easy to explain and to understand. It has the positive effect that increased braking will also increase the total consumption which makes it hard to cheat the system. The regeneration will not be more than the total energy consumed in these events.

Different parameters for different vehicle type

The parallel hybrid and the serial hybrid have their own specific characteristics, which makes it necessary to use different sets of measuring parameters for the different vehicle types. We will explain a little about the challenges and possible solutions for each vehicle.

Parallel hybrid

For parallel hybrids, a two new grade criterias can be implemented:

  1. Electric distance
  2. Dynamic consumption in low speeds.

Electric distance

The electric distance is based on how much of the driven distance is electric engine drive and how much is driven on fossil fuel. The electric distance can be defined as the distance driven on electricity divided by the total distance, to get a percentage, where the grade will be better, the higher the percentage figure is. The limits have to be configurable to accustom to different circumstances.

Three parameters could be combined to become the electric distance.

Electric distance parameters:

  1. Coasting, which is distance driven without the engine drives the wheels.
  2. Recharging, which is the distance the vehicle is driven with negative fuel consumption. Depending on model, recharging can occur while coasting as well.
  3. Electric engine propulsion, which is the time when a parallel hybrid is driven solely by the electric engine.

These three parameters summed up and divided by the total distance driven, makes up a good base for the electric distance criteria.

Dynamic consumption in low speeds

Part from shutting the combustion engine off while idling and regenerating energy when braking, one of the main features of a parallel hybrid is that it can accelerate up to a certain speed with the electric engine, which in saves fuel.

However, if the combustion engine have to kick in too early because of aggressive driving, this has to show in the eco-driving model as dynamic fuel consumption. The fuel consumption under a certain speed will therefore affect the fuel economy grade negatively.

Serial hybrid

A new category is under development for serial hybrids:

Acceleration.

The acceleration grade model is at an early stage, and needs more investigation before it is ready to launch, but we will discuss possible solutions and options here.

The acceleration category is a measure of how optimally the serial hybrid has used the stored energy in the supercapacitor. The supercap can only deliver a certain amount of energy in a short period of time. When this amount of energy is not sufficient for the required acceleration, energy is taken directly via the generator, powered by the combustion engine.

Electric distance or charging drive not useful

The serial hybrids’ special function is not suitable for measurement of electric distance, since all driving is electric engine drive. Theoretically, one could count positively on charging drive. The problem is that if the battery is replaced with a supercapacitor, this has a limited size, and at least as important as charging the supercap is to utilize the existing charge optimally when accelerating.

Acceleration grade

This speaks for an acceleration measurement. To make an acceleration grade fair, it should be completely disconnected from the charging level. A simple assumption here is that a smooth and even power outtake is the best way to utilize existing charging. This could probably be simplified to smooth acceleration, without becoming a noticeably less valid parameter. Then a limit for acceleration could be used to calculate a dynamic fuel consumption value. The acceleration grade should be presented as a loss of energy. The easiest way this is achieved, is when acceleration exceeds a limit, the fuel consumption is measured and recorded as a loss.

Power outtake hard to use

Alternatively, you could calculate power outtake. The problem with this method is that the power outtake increases with speed, at the same time as the ability to influence the power outtake decreases, so it is probably better to measure the acceleration.

Conclusion

Eco-driving measurements for electric vehicles and hybrids are still at an early stage, and there are several fields in need of further investigation and trials. However, already now, the models are good enough to motivate investing in eco-driving systems for hybrids and electric vehicles.

Electric vehicle breaking point

When the landslide of conversion over to electric vehicles comes, it might happen very rapidly, because when the breaking point where electric vehicles with the same range as combustion engine vehicles become the cheapest alternative, then the market will tip over and the combustion engine might quickly become obsolete. Then the infrastructure must be ready in all fields, where eco-driving is one of the important fields. Eco-driving also has the potential to advance the breaking point, due to reduced energy consumption which leads to longer range for electric vehicles.

Disclaimer: The content of this blog post is the author’s opinion and doesn’t reflect the opinion of any other person or organisation.

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