Electric Vehicle – Maximising range by design

When tapping into the discussions on a hybrid, or electric vehicle, you often see the term ‘hypermiling’. What is this exactly, how does it work and how do you achieve it?

 

Very simply put, hypermiling is getting the most range from your car given a single charge/tank of energy in any form. Usually the manufacturer provides a figure that describes the amount of kilometres or miles that can be driven with a full battery or tank. For example, the Nissan Leaf can do 160 kilometres / 100 miles on a full charge. These figures are provided by the manufacturer and are derived from tests using very general driving conditions. If you can somehow drive in a more efficient matter, you can get more range from your single charge.

 

Basicly there are two areas to focus on to get the most range from your charge. The first is in the engineering phase, when building the car. The other is when you have a vehicle and drive with it. This last one is the actual hypermiling, the first is related, but you can learn a lot from the engineering knowledge.

 

When building a vehicle with the aim of driving as far as possible with it on a single charge or tank, there are two main focus areas: weight and efficiency. Weight is an important factor, the less your vehicle weighs, the bigger the range is you can achieve. Efficiency is the other factor, the more efficient your drive train is, the more range you can get.

Weight

 

A lot of car manufacturers spend a lot of time and effort on reducing the weight of their vehicles; carbon fibre body work, aluminium rims and other solutions are often mentioned when they promote their newest light weight vehicle. Other nice solutions that I’ve personally read about was taking out the spare tire and providing a can of repair-gel. If you get a flat tire, you can repair it by spraying the can empty in the tire. The can obviously weight a lot less than the extra tire and there is even the added extra bit of room (in case you are building your own vehicle: that is extra battery space for you!).

The carbon fibre body of the Tesla Roadster ensures a low total weight

 

Other approaches all boil down to: do I really need to take this along every trip? Is there a lighter alternative? A last example in this can be borrowed from the more luxury SUVs, where it is often possible to remove some of the chairs in the back. Not only provides this more room for something else you might haul along, but it also saves you the weight of these chairs. Again for the folks considering building their own (hybrid/electric) vehicle, give it some thought and see how much you can save by taking the seats in the back out (and being able to put them back in if you are travelling with four people again.

Efficiency

 

With regards to energy efficiency, the drive train is the other focus of attention when trying to get the most miles from your vehicle. Efficiency in this case is related to the conversion of energy (electricity or fuel) to the work that can be performed with it (driving from one place to another). Less efficiency means less range, more efficiency implies a better ‘conversion rate’ and gets you the most range. As an example, let’s take a look at the battery in a car. The battery has a charging efficiency, but also a discharging efficiency. Simply put, you lose a bit of energy if you charge the battery and you lose another bit if you draw power from it. Generally with decent batteries these efficiencies are quite good. As a “rule of thumb”, let’s assume both efficiencies are around 90% (better and more expensive batteries can do a lot better). With every charge or discharge, around 10% of the energy involved is converted into heat for example. The efficiency is highly dependable on the amount of power you use; the efficiency becomes better at low powers and you have higher losses at high power. This is also why you can cover a larger distance by driving slowly!

Each engine has a "sweet spot" with the highest efficiency

 

Another example is on an internal combustion engine. We all know that the electric motor has a far higher efficiency, but in a hybrid the internal combustion engine is often used as the main power source. Either to directly provide (mechanical) power to the wheels, or via a generator provide electricity to top up the battery. The internal combustion engine has one point of operation (at a certain RPM it provides a certain torque) at which it has the best efficiency. In case of an internal combustion engine used to top up the battery, often the engine is operated at this highest point of efficiency to generate electricity. That is of course, if the extra power that is generated is actually required.  

 

Summary

 

To sum things up, what the car manufacturers do is get the lowest weighing car with the most efficient drive train. If you are busy building your own hybrid/electric car, get at least decent efficiency components for your drive train (an electric car that is only 10% efficient is definitely not sustainable). Also look at what you can do (sensibly!) to reduce weight, but don’t forget: Safety First! Driving around with four wheels, a super efficient drive train and a comfy chair on top might get you a lot of range, but is hardly safe, or sexy.

 

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Sources: Autoblog Green on Tesla EETimes on Hybrid Vehicles