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Diesel or Hybrid or Petrol?

A personal perspective...

by Julian Edgar

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I’ve just stepped out of my 1995 Peugeot 405 SRDT turbo diesel sedan, having driven it 800 kilometres in two days, making a holiday trip loaded with a bootful of luggage and my wife and 3-year-old (no, they weren’t in the boot).

When I arrived home, I saw sitting the driveway my 2001 Honda Insight hybrid and my wife’s 1998 Toyota Prius hybrid – the latter’s the one with the defective high voltage battery. As a motoring journalist, I have also driven current model Volkswagen, Peugeot, Audi and Hyundai diesels, the current model Prius (in fact I drove that car across half of Australia and back) and the Honda Civic Hybrid. So, unlike most who seem to write on this topic, I have personal ownership experience of hybrid and diesel cars as well as test car experience.

So, which automotive engineering approach is best – conventional petrol, hybrid petrol/electric or diesel? Listening to some people you’d figure, with equal strength of belief, that:

  • Hybrids are just wanky cars for those who like to appear green

  • Diesels are slow, noisy and polluting

  • Hybrids are for city and diesels are for country

  • Conventional petrol engine cars can do all that either diesels or hybrids can do – and at a much lower cost

In fact, I think every one of these statements is rubbish. Why? Let’s take a look at the misconceptions.

  • Hybrids are just wanky cars for those who like to appear green

If the quality of the car was symptomatic of the quality of the owner, only dickheads would ever have driven rotaries, all WRX owners can’t drive and late model Monaro drivers are all in their sixties and are driving the cushy thing they deserve.

Stereotyping owners is stupid automotive engineering analysis.

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Maybe people buy the Prius because they want to look green – but who cares? Maybe government departments buy the Prius because they want to be seen to be doing something environmentally friendly – but that has nothing to do with the green credentials of the car itself.

The bottom line is that on the Australian Government’s Green Vehicle Guide, the top performer in terms of fuel economy and emissions is the Toyota Prius.

So the most impartial guide to cars in Australia lists the Prius as the best 'green' new car you can buy. Full stop.

  • Diesels are slow, noisy and polluting

Diesels don’t need to be slow – the European manufacturers (eg Audi and BMW) are now selling in Australia very powerful turbo diesels, ones no one would suggest are slow. However, if you develop a lot of power in a heavy car, that performance is paid for at the pump - irrespective of whether that pump flows petrol or diesel.

So it’s true to say that the best performers in fuel economy (and so, consequently, in greenhouse gas emissions) are not fast cars. That’s exactly as you’d expect, is the same as it’s always been, and is the same as you’d expect it to always be! (Maybe in the future, short-range hi-po electric cars excepted.)

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Noise? Diesels certainly have a characteristic clatter audible from outside the vehicle – for a roadside pedestrian, picking a diesel from a petrol engine is pretty easy. But inside the car, noise levels are now much the same for petrol and diesel. The note may be a little different, but the sound pressure level is in the same ballpark.

But it is in pollution where the statement holds the greatest truth. Diesels, intrinsically, have higher polluting levels of oxides of nitrogen and particulates. The latest diesels are vastly better in these regards, but they still struggle against the quality of petrol engine emissions, let alone hybrid emissions. Band-aids such as the widely-billed Mercedes BlueTec treatments are all well and good, but the emissions road for diesels is still steeply uphill.

  • Hybrids are for city and diesels are for country

The idea that hybrids are for the city and diesels for the country seems to have gathered currency, even amongst those professional engineers and scientists who should know better. Why should they know better? Well, it depends on the vehicles in which the respective drivelines are placed!

In a lightweight, small and aerodynamic car, a hybrid can get excellent open-road economy. In a large, powerful and heavy car, a diesel can get relatively poor open road economy. What people might believe they are saying is that, in the same vehicle and with the same power, a diesel will give better country road results than a hybrid. That may in fact be the case... except we don’t currently have cars that fit those criteria.

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When AutoSpeed drove a current model Toyota Prius from the Gold Coast to Adelaide and back we gained 5.3 litres/100km for most of the trip. When we did a long country drive in a similar size Peugeot 406 diesel turbo (a car that gets better economy than the current larger and heavier 407 model) we got a poorer 5.9 litres/100km.

What people really mean is that in city conditions a hybrid will simply kill a diesel for economy, whereas on the open road, it will be a closer match. That’s a very different statement to the one highlighted in bold above.

  • Conventional cars can do all that either diesels or hybrids can do – and at a much lower cost

People love making comparisons of Prius fuel economy to the economy of the Ford Festiva, Daihatsu Handi Van, Honda Jazz and a host of other old and new small economy cars. Trouble is, the Prius is in a completely different class in terms of interior space, performance, safety, features and comfort. Take each of these criteria into account, then make a valid comparison to another car, and the outstanding fuel economy results of the Prius become clear.

I’ve even seen people say the Prius is vastly too expensive – but AFAIK, when it was released in I-Tech form, it was the only car in its price range sold in Australia with sat nav... to name just one feature! (I therefore think it should have come down in price subsequently as other cars have marched forwards in features – or, alternatively, the next Prius should be cheaper.)

Re diesels? Where the same car is available in both diesel and petrol guises, the diesel always wins on fuel economy – always. The Mazda 6 is a good example, 5.9 litres/100km for the diesel versus 8.8 litres/100km for the petrol. (But as is nearly always the case, the performance of the two cars isn’t identical.) And cost? The diesel Mazda 6 is actually cheaper...

Reality

OK, having dispelled some of the myths, let’s look at some of the realities.

  • Hybrids’ High Voltage Batteries

Firstly, those people who have said that the length of life of the high voltage batteries in hybrids will prove to be their Achilles Heel are absolutely right. The Japan-only NHW10 Prius, released in 1998, is now experiencing widespread battery failure. My wife’s car, a grey market import, has a defective battery. Battery repacking is not available and even getting secondhand batteries is difficult – a difficulty made, it must be said, much more so by its grey market status.

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Both Toyota and Honda are bending over backwards to replace high voltage batteries in dealer-sold hybrids – in the US it is official policy to replace, free of charge, Insight batteries of cars sold in certain, climatically hot areas. In Australia Honda won’t tell you that you can have a new battery - but they fitted one to my Insight at no cost.

The next two Prius models – the NHW11 and NHW20 – have better designed batteries but it is an absolute certainty that these battery packs will not last the mechanical life of the car. (That’s especially the case when the Prius mechanicals seem exceptionally durable!) Unless aftermarket manufacturers take up the challenge to produce OE direct fit batteries at reasonable prices (and it’s quite likely that in the future they will), or car manufacturers dramatically drop the price of new batteries, then hybrids will adopt orphan status as they get old.

  • Hybrid and Diesel Performance

People who complain about performance of economical hybrids and small turbo diesels have usually never driven them. Both types of cars are usually not overly endowed with power but usually have very strong bottom-end and mid-range torque curves. Another way of putting this is to say that they both make lots of power low in the rev and speed ranges.

Drivelines with strong bottom-end performance are easy and quick to drive in city and suburban conditions, something which most of us spend most of our time doing.

They do not have high amounts of power necessary for high-speed hill-climbing or setting good 0-100 km/h times. In both hybrids and diesels, these deficiencies are easy to remedy – just as with any car, fit a more powerful drivetrain. However, again just as you’d expect, economy then goes backwards.

All the passenger car hybrids and turbo diesels (but not the non-turbo diesels!) I have driven have had quite adequate power for normal use by a decent driver.

  • Real World Fuel Economy

It’s an old chestnut but the actual fuel economy that is gained from a motor vehicle – conventional, hybrid or diesel – depends a lot on how it is driven. So there will always be unhappy Prius owners whose fuel consumption is (comparatively) lousy, there will always be diesel owners who complain incessantly and there will always be petrol car owners who get mind-bogglingly good fuel economy.

I can state with absolute certainly that all the hybrids and diesel passenger cars that I have driven have got excellent fuel economy, better in every case than a similar size/weight petrol engine car.

I’ve also gained very good economy from conventional cars – especially Hondas – but even taking that into account, diesels and hybrids have been better.

Be wary of those people who might be very economical drivers of conventional cars, comparing their economy figures with people who drive hybrids or diesels with little understanding of economical driving....

Conclusion

So which way do I lean – diesel or hybrid? (I think that conventional petrol engine cars are now right out of the race for economy and emissions.)

The market diversity is currently too small to make any such long-term decision viable. There are no commercially available diesel hybrids. The world’s best-selling hybrid – the Toyota Prius – uses what can now only be described as an old, low tech petrol engine. (It’s a 10 year old ex-Echo engine with altered valve timing and a low redline – that’s all.) The Honda Civic hybrid is hamstrung by the primitive way it mixes electric and engine power. In the local market, diesels are going backwards – best exemplified by the Peugeot 405 > 406 > 407 progression, where power and weight and fuel consumption (and, it must be said, safety) have risen with each model.

The regenerative braking and engine-off pure-electric propulsion of a hybrid in city traffic is not only highly efficient, it’s bewitching. A turbo diesel engine’s torque simply belies its swept volume – and the diesel is gaining more useful energy from every explosion of fuel/air mix than a similar size petrol engine.

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But on rough roads the Honda Insight rides like a truck; in strong side winds the Prius wanders like a drunken sailor. The US-market SUV hybrids look as silly in rationale as putting aerodynamic spoilers on a military tank. The Audi A6 3-litre diesel has a government test fuel economy of 8.7 litres/100 – good compared with the petrol opposition but in itself, nothing special.

On my available budget I have chosen to buy a Honda Insight – a car that gives better open road fuel economy than any other that has ever been sold in Australia. I can easily get 3.0 litres/100km on the open road.

I have also bought a grey market import Prius, one that I subsequently turbocharged. In my driving it gets about 6.0 litres/100 in its modified form. However, without a viable high voltage battery, it currently doesn’t go anywhere.

And now I have a diesel Peugeot. Despite my comments above, I would have bought a 406 diesel, simply for the better safety and much more high tech common rail diesel engine. But my budget doesn’t extend that far and so I have the older 405 turbo diesel. And on my 800 kilometre trip of the last two days, it got 5.5 litres/100km.

In any other similar size car in my price range, the economy would have been 30 per cent or so worse. Except of course from a Prius!

See also Diesel vs Hybrid - the Battlelines are Drawn

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