Electric vehicles - energy use (3)

Regardless of its type of fuel, the energy drawn by any road vehicle is a function of three main factors: air drag, accelerating and braking, and rolling resistance.

The Tesla 3. Pic: Tesla

Air drag

This relates to frontal area and aerodynamics and particularly to speed (energy use rises with the cube of the speed). It is thus also affected by the prevailing wind. This is not usually a major factor in most countries but it is very much so on Australia’s 1675 km (141 miles) Eyre Highway. Often called the Nullarbor, the highway links South and Western Australia and is very close to the ocean for much of the way. That wind tends to be either from in front or behind and can be as high as 30-40 km/h. If driving into the 30 km/h wind at 90 km/h that’s a battery flattening 120 km/h.

Wind resistance is a powerful reason for driving anticlockwise around Australia such that one drives north around September, around the top during winter and back down the West coast and to where one started in late summer. This should result in a following wind for the west and east crossings.

Electric-only vehicles of today are most suited to urban driving, but as battery technology inevitably advances, and charging facilities increase, these will be decreasing issues.

The (2016) Chevrolet Voltec electric vehicle motor and transmission. Pic: Chevrolet. 

Acceleration & braking

The energy involved in acceleration and braking relates substantially to the laden weight of the vehicle. Whilst existing batteries are far heavier than their range- equivalent of petrol or diesel, an electric vehicle motor and transmission is much simpler and lighter. It is also 80% to 90% efficient (a fossil-fuelled engine is only 25%).

BMW i3 ultra-light carbon-fibre body shell saves weight. Pic: BMW.

Body shells can be made much lighter: BMW’s i3 electric car has an ultra-light carbon-fibre body shell - cancelling out much of the battery weight.  That extra battery weight, however, is expected to be only a short-term issue. 

As our article Electric Vehicle Batteries notes, huge efforts are in progress worldwide to reduce the weight of rechargeable batteries. This will also enable a longer range between recharging. 

Rolling resistance

Rolling resistance is directly proportional to minor friction losses, minor heat loss due to tyre wall deflection (<3%), and speed. There is thus no increase or decrease in an electric vehicle’s rolling resistance. There is, however, the considerable energy advantage of electric (and hybrid) vehicle over combustion engine vehicles - of simple and effective regenerative braking. This recovers the kinetic energy that would be otherwise lost in heat-generating braking by an electric car’s motor momentarily acting as a generator and charging the batteries. 

Stop/starting in traffic

In recent years, petrol and diesel engine cars have a (usually optional) engine stop/starting system for use in congested traffic. Whilst this saves fuel, electrical energy is used for each restart. Here, electric cars will have a considerable edge as no energy is drawn whilst at rest, nor extra when restarting.

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