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Assessing the Alternatives

The most important story we'll run this year

by Julian Edgar

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At a glance...

  • Assessing the energy efficiency of different fuels and engines
  • Assessing the greenhouse gas emissions of different fuels and engines
  • The best evaluation of alternatives we've ever seen
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Even those with only a cursory interest in automotive technology can see the changes looming from just beyond the horizon – the revolution that will either (depending on your perspective) enlighten or threaten us.

BMW currently has their hydrogen-powered 7-series doing the world rounds of breathless journalists; Toyota’s hybrid petrol/electric Prius has achieved a success that the company could only have dreamt of; and electric cars like the Tesla Roadster look set to change the automotive paradigm.

On the fringes are bio-diesel and ethanol and CNG and LPG.

And not for one moment to be underestimated are conventional petrol engines - but boasting efficiency improvements like direct fuel injection and turbocharging.

Listen to the hydrogen exponents and you’d think there’s the answer. Listen to the biodiesel enthusiasts and there’s no doubt – we all need to use that renewable fuel. But then again, the arguments of those that believe in hybrids seem persuasive....

So how on earth do we make sense of all this? What are really the best answers – the automotive technologies that will give the optimal outcome in terms of energy consumption and greenhouse gas emissions?

A seminal paper written by Andrew Simpson draws back the cloud of obfuscation, vested interests and often pure ignorance that confuses these most important of issues. Produced when the author was working for the Sustainable Energy Group at the University of Queensland, the paper in one fell swoop undercuts many of the ideas we’ve started taking for granted.

The data is absolutely vital reading for anyone interested in where automotive technology should be heading.

Making Sense of it All

For the first concepts to get across are ‘Well-to-Tank’ and ‘Tank-to-Wheel’ figures.

  • Tank-to-Wheel

When we asses vehicle efficiency, we most often talk about the fuel consumption of the vehicle. For example, an efficient car might be able to travel 100 kilometres on 5 litres of petrol. We call the resulting fuel economy 5 litres/100km - and reckon it’s pretty good. (And it’s probably also going to be good in terms of greenhouse gas emissions, too.)

This figure can also be termed the tank-to-wheel efficiency: that is, how much of the energy of the fuel in the tank makes it to the wheels as propulsion for the car. In these terms, a diesel engine car is more efficient than a petrol engine car; a 5-litre V12 engine car is less efficient than a 2 litre 4-cylinder.

But the concept doesn’t apply just to internal combustion engines. Electric motors are highly efficient (vastly more so than internal combustion engines) and so the tank-to-wheel efficiency of an electric car is likely to be very high. (In this case, the ‘tank’ is the battery.)

Another car with a high ‘tank-to-wheel’ efficiency is a fuel cell car, a car that uses the chemical reaction in a special stack to produce electricity that in turn powers an electric motor.

But in terms of overall efficiency, the tank-to-wheel figure is often a complete furphy, misleading and sometimes downright false.

Why? Read on...

  • Well-to-Tank

Clearly just as important as the Tank-to-Wheel efficiency is the Well-to-Tank data.

In other words, how much energy does it take to refine the fuel so that it is suitable for going into the car’s tank?

Think of a petrol engine car. Petrol is not able to be dug straight up out of the ground and labelled with an octane rating. Instead, it needs to be laboriously located, obtained using energy-intensive giant oil rigs (either land or sea), piped and transported to plants where it is refined into the substance that we know as 98 RON unleaded. Each of these steps takes energy and releases greenhouse gas emissions.

Compare that with (say) a wind generator producing electricity. The wind is free in terms of both energy and greenhouse gas emissions. Capturing that wind energy as electricity requires a generator and tower and rotor; but after those have been paid for, the energy is free of encumbrance. Then, even if the wind generator itself is only 20 per cent efficient, getting the electricity into the battery that powers the car (the “tank”) will be very low in energy and greenhouse gas cost.

So the well-to-tank figure takes into account the on-going cost of energy and greenhouse gas emissions for producing the fuel the car runs on.

If any sense is to be made of what is best for all of us, clearly it makes sense to include both the Tank-to-Wheel and the Well-to-Tank figures – done by using a Well-to-Wheel figure.

  • Well-to-Wheel

The Well-to-Wheel figure is the whole box and dice: the total energy and greenhouse gas emissions cost of turning the wheels of your car. It is – to excuse the expression – the no bullshit evaluation of the best approaches to powering the cars of today and tomorrow.

The Well-to-Wheel figures take into account not only the efficiency of the device powering the car but also the way in which the fuel to power that engine has been produced.

Without that background it is simply impossible to evaluate the real situation. If hydrogen fuel cell electric cars produce little emissions and run for a long time on their tank of fuel, does that make them the panacea? Not if producing the hydrogen fuel in the first place takes enormous energy... Do biodiesel cars make a lot of sense if the burning of the fuel adds little total CO2 to the planet? That depends on how the fuel is produced in the first place...

Think about it for a moment and you’ll realise that the Well-to-Wheel figure is an absolutely vital parameter in evaluation of alternative car technologies.

First Half of the Story

Let’s look at what will be most familiar – the energy-efficiency (and greenhouse gas emissions) of the tank-to-wheel process. That’s as simple as how much energy of the fuel in the tank (and remember the “tank” might be a battery) gets to the wheels, assessed in equivalent litres/100km of petrol. (In other words, the total energy consumption over the test expressed in terms of petrol per 100 kilometres.)

Andrew Simpson’s paper was first published in 2003, with updates in 2004 and 2005. We have communicated with the author and he believes that the results are still valid today (2008). His benchmark was a 2003 model Holden VY Commodore. The Commodore, when assessed over the New European Driving Cycle, had a fuel consumption of 10.1 litres/100km.

Click for larger image

Andrew Simpson evaluated no less than 32 other forms of propulsion (click on the graph to enlarge). Importantly, these alternative cars were modelled to have the same performance and range as the Commodore.(The exceptions to this are the nickel metal hydride and valve regulated lead acid battery electric vehicles where it is not technically feasible that they have a driving range matching the Commodore.) The fact that most of the vehicles matched the Commodore in terms of performance and range is very important because this ‘level playing field’ is seldom adopted when comparing alternative technologies. For example, this criterion meant that the battery electric cars were relatively heavy – as they would be in the real world.

Rather than look at all the alternative technologies modelled by Andrew Simpson for their tank-to-wheel performance, let’s look at those most in the news.

As already indicated, the Commodore scored 10.1 litres/100 in the standardised test (we wonder if the current VE model would do as well!). An LPG-fuelled equivalent internal combustion engine Commodore was barely any better at 9.7 litres/100, while the best performance (8.1 litres/100) using an internal combustion engine was when it was fuelled with diesel. Note that biodiesel (8.2) was actually slightly worse than conventional petroleum diesel. This makes sense when you consider that the energy density of biodiesel is lower than that of conventional diesel.

Then come the hybrid internal combustion engine / electric cars – one running on petrol turns in a 7.7 litres/100 figure and a diesel fuelled hybrid would get 6.9 litres/100.

(It’s worth being reminded at this point that all cars have the same performance and range.)

Fuel cell electric vehicles? Well, if they drink petrol, they’re terrible. But on hydrogen they’re much better – as low as 5.1 litres/100km.

But best of all are the pure electrics – remember, their electric motors are very efficient and so nearly all of the battery juice ends up pushing the wheels – an energy equivalent of as low as 3.5 litres/100km for a lithium-ion battery electric vehicle (and the Li-Ion car has the same range and performance as the Commodore).

So, in really general terms, the ascending order of merit for the Tank-to-Wheel figures go:

  1. Petrol internal combustion engine

  2. Hydrogen fuelled internal combustion engine

  3. Diesel

  4. Hybrid petrol electric

  5. Hybrid diesel electric

  6. Hydrogen fuel cell electric

  7. Battery electric

But as we’ve already mentioned, that is not the full story – in fact it’s only half the story!

The Full Story

Rather than looking at the well-to-tank figures and then correlating them with the tank-to-wheel figures described above, let’s jump straight to the vital stuff: the well-to-wheel figures. The figures that really matter, that take into account both tank-to-wheel and well-to-tank data.

Click for larger image

In this diagram (click on it to enlarge) the Commodore is normalised to “1” – anything above that is worse and anything below that is better.

The first shock is there are approaches that are worse than the benchmark - considerably worse. Using coal to produce either liquid or gaseous hydrogen that is run in a fuel cell car has an energy consumption that is between 15 and 46 per cent worse than the Commodore on petrol. In greenhouse gas emissions they’re between 35 and 75 percent worse!

That’s a really vital point to keep in mind: despite the high technology of gasifying coal and then running a fuel cell hybrid electric vehicle, the result is worse than the status quo.

Using a coal-powered electrical generating station to charge a lithium ion battery electric car also gives results which are at best marginal – fractionally better energy efficiency than the Commodore but about 17 percent worse greenhouse gas emissions.

So here’s another vital point. Even a battery electric car, despite its high tank-to-wheel efficiency, is poor overall if it uses a coal-powered generating station for its juice.

Another approach that is worse than the petrol powered Commodore is using a methanol fuel cell hybrid electric vehicle, where the methanol is chemically refined from natural gas.

So what is clearly better than the Commodore? Hybrid petrol electric and diesel electric cars are better than the Commodore (and very similar to each other), while hybrid LPG cars are better again.

But – as you would expect if you’ve been paying attention all this time - it is the renewable energy fuelled cars that do best overall. The winner is the renewable energy powered (eg solar or wind) lithium ion battery electric car – it has no greenhouse gas emissions and the best overall energy usage of the lot, using only 33 percent of the energy of the Commodore.


This story is most important in highlighting what doesn’twork as much as what does work. Despite impressive high technology, some approaches (including those being touted as panaceas by some major car companies) have appalling energy and greenhouse gas results. Others that may look the same (eg battery electric cars) can have completely different results, depending on how that electricity is generated.

Andrew Simpson writes (with our emphases):

  1. The best way to utilise an energy feedstock is via as direct a pathway as possible, avoiding unnecessary energy conversions. This is an important conclusion in regard to synthetic fuels such as hydrogen. For the pathways considered in this study, it is preferable not to use hydrogen since the energy losses and emissions incurred in the production of hydrogen outweigh the higher efficiency of fuel cell-based powertrains.

  1. Use of coal as a feedstock for production of vehicle fuels will result in extremely high levels of full cycle energy consumption and greenhouse emissions.

  1. Hybrid Electric Vehicles using conventional fuels (petrol or diesel) offer significant near-term reductions in energy intensity and greenhouse gas emissions.

  1. Natural gas appears to be a promising transitional energy feedstock for automotive fuels. Hybrid Electric Vehicles fuelled with Compressed Natural Gas or Liquefied Natural Gas offer even greater reductions in energy intensity and greenhouse emissions than Hybrid Electric Vehicles using conventional fuels. Natural gas-fired electricity can also be used to charge electric vehicles resulting in the lowest greenhouse emissions of any natural-gas pathway.

  1. Based on their energy intensity, biofuels may not be the most practical method for reducing greenhouse gas emissions.

  1. Well-to-wheel pathways based upon renewable electricity generation offer near-zero greenhouse gas emissions. However, renewable electricity should be utilised directly in an electric vehicleto avoid the energy intensity of converting it into other fuels (i.e. hydrogen).


  • Fuels

ULP – unleaded petrol

LPG – liquefied petroleum gas

CNG – compressed natural gas

LNG – liquefied natural gas

GH2 – compressed hydrogen

LH2 – liquefied hydrogen

MeOH – methanol

EtOH – ethanol

  • Vehicle Types

ICV – internal combustion engine

FCEV – fuel cell electric vehicle

HEV – hybrid electric vehicle

BEV – battery electric vehicle

FCHEV – fuel cell hybrid electric vehicle

  • Battery Types

Li Ion – lithium ion

NiMH – nickel-metal hydride

VRLA – valve regulated lead acid

Well-to-Wheels versus Cradle-to-Grave

A well-to-wheels study considers only energy inputs/emissions outputs used for driving. But what about the energy used (and the emissions created) when the car is being manufactured? And what about the emissions and energy for development of the fuel creation infrastructure (whether that’s a power station or an oil rig)? These latter factors are taken into account only in a ‘cradle-to-grave’ analysis.

Andrew Simpson says: “I did not include the ‘embodied energy/emissions’ (as they’re known) in my study.

“Ideally, you should include everything, but as a practical matter, you have to set the system boundary somewhere.  A rule-of-thumb that I have seen is that the embodied energy in making a car is ~10% of the energy it consumes via driving over its life.  This of course varies by technology, and doesn’t mean you can ignore the embodied factors, but I think my comparison was informative nonetheless.”

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