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How green is a diesel generator?

Wednesday, October 21st, 2009 by

There is a reason why we’re the only team driving a pure electric vehicle (not a hybrid or alternative liquid fuelled vehicle) in this years’ “Eco Challenge” event.

Hidden Valley Racktrack in Darwin

Hidden Valley Racktrack in Darwin

Right now, there isn’t an EV charging infrastructure on the road from Darwin to Adelaide.

So the generator set we’re using is a simulator – its going to be pre-positioned ahead of us each day, to create the mobile version of the sort of high power charging infrastructure that is already turning up at key recharging points in California (the home of the Tesla Roadster).

Hence, its important to understand that we’re not trying to say that using a high capacity generator set is a ‘green’ way to run the Tesla Roadster. Again, its a simulator.

This exercise is about seeing how well the car does with its energy consumption and energy efficiency – in anticipation of the emergence of widespread electric car charging infrastructure (yes, even in Australia) over the next several years.

While we’re going to be carefully logging everything – including the amount of diesel the genset needs to charge the car (and for interest, that should lead to a ‘diesel litres per 100 km of Tesla driving’ figure) – the thing we’re really looking to measure is the performance of the car, against its manufacturers’ claims for power consumption (measured, most commonly, in ‘Watt-Hours per Mile’ or ‘Watt-Hours per Kilometre’ (Wh/Km).

A “Watt Hour” is quite literally one watt of energy for one hour. For instance, if you turn on a 100 Watt light bulb and leave it on for one hour, that’s 100 Watt-Hours of energy (or 0.1 Kilowatt Hours).

To put this in proportion – the Tesla, travelling at a constant 60 kilometres per hour (according to Tesla Motors figures) should consume a little over 100 Watt Hours per Kilometre. So one kilometre of road running at 60 km/h consumes around the same energy a 100W lightbulb needs for an hour.

And the size of the ‘tank’ in a Roadster is nominally 55,000 Watt Hours (55 kWh). Doing the obvious sums (55000 divided by 100) means that if driven at a constant 60 km/h on a level road, the car should be able to travel about 550 kilometres on a single charge.

Drive faster – and the range reduces (just like a petrol car). Drive slower, the range increases (just like a petrol car).

But of course, we don’t drive our cars at a constant 60 km/h on level ground. So the official driving range of the Tesla Roadster on a full charge (under a USA standard driving cycle assessment) is 392 km (244 miles). That assessment – the same applied to petrol cars in the USA – attempts to address real world driving situations to come up with a reasonable example range number that can be compared between cars.

Tesla Roadster Range vs Speed Graph. Click for full Tesla Motors Article

Tesla Roadster Range vs Speed Graph. Click for full Tesla Motors Article

The Eco Challenge also exists to address the obvious question here –  the manufacturer has claims of what the car requires, in terms of energy, to drive it along the road. But that is in theory – and even the USA driving cycle assessment is a standardised simulation run.

So how about the real world?

Does the car need more or less energy than Tesla Motors claim it does? How much does careful driving help? What happens when its (really really) hot outside? When its hilly? When it rains?

This is why “Eco Challenge” is not a race (the Solar Cars are racing over the same path, but we aren’t). The Eco Challenge tests vehicles against manufacturers claims, in the real world, over a very large distance, through varying conditions, under strict supervision.

Not surprisingly, one of the award categories for the event is judged based on the percentage improvement achieved by drivers over the manufacturers’ claimed figures.

When we arrive in Adelaide, we’ll know what the Tesla is really capable of. We will have measured it in all sorts of real world conditions, up and down and sideways. I can’t wait!

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