Let’s say you are a big-shot. You need to get around. And public transport is just not going to cut it.

You do a lot of miles and want to go the extra mile to minimise your impact – what should you do?

The answer any good car salesman would give you is: get a Prius (in case you missed it, this is Toyota’s flagship ‘hybrid’ high efficiency offering).

Sounds good, but what is a a hybrid? Is that electric? Do I need to plug it in at night? And what are the alternatives? Are there any?

In this section, I hope explain this and some of the other things coming down the turnpike.

Electric, Hydrogen and Hybrid Cars

Time to plug in?

Electric cars are not new. The idea has been around since cars were first made. The problem has always been with the batteries – you need big ones to go far, but the bigger they are, the heavier the car, and so the more batteries you need.

Add to this that batteries take hours to “fill up”, whereas you can can fill the petrol tank in about a minute, and you start to see the issues.

Once you point out that electrical energy is not exactly cheap, and that you are probably not saving the planet unless you get your power from a hydro-electric dam, and you can see why they have not taken off.

So why don’t they just make smaller batteries that charge-up quicker?

Well they are trying. The cell phone industry has driven up the ‘energy density’, but batteries are still rather pathetic compared with petrol. While 60 litres of petrol contains about 2GJ of energy, a pretty good (NiMH) battery of similar weight will muster a paltry 0.02GJ (less than 100th as much). The very highest-tech batteries – still impractical – such as Lithium Thionyl Chloride (LiSOCl2), still contain less than a 10th the energy of simple gasoline.

As a result electric cars cannot go very far on a single charge, and makes them impractical for most road users.

The smart money has not been holding its breath. The next beacon in the night was hydrogen. It packs in three times more energy in per kilogram than petrol. It makes water when it burns. How lovely.

The first problem with hydrogen is the overall efficiency – first you need to make electricity (with some inefficiency losses) – then you need to ‘make’ the hydrogen (with further losses), then you need to compress the hydrogen (yet more losses) and then you need to either burn it or convert it back into electricity in a fuel cell – both of which have more losses still. They all add up. However, these efficiencies are perhaps temporary problems…

A more permanent problem with hydrogen is its low density and thus large volume. Even when compressed to a liquid it still only contains a third the energy of an equivalent tank of petrol. Oh, and because its under very high pressure, the tank needs to be made from thick steel.

This means hydrogen cars also have poor range – and as hydrogen filling stations are rather rare, again this technology is still far from practical for Joe Public.

Due to these problems, the smart money has been looking elsewhere: a compromise solution, that tries to improve efficiency and also reduce emissions but without the short range: the so-called “hybrid”.

The Next Prius - the Toyota Prius is the top selling Hybrid

This is not rocket science. It is basically a car with a fairly normal engine, but also with an electric motor and battery pack.

The logic is that regular internal combustion engines waste energy when the car is running slowly or stopped at the lights. So why not switch over to an electric motor in these situations? And the electric motor runs off batteries which charge from the regular engine when it is running, though in some models the batteries can get a supplementary charge via a power socket at home .

But there is of course something extra that electric (and hybrid) cars can do: they can recover energy during braking – they call it regenerative braking.

The electrical boffins among you will know that a motor, which turns electric current into rotary motion, can work the other way too – turning rotary motion into electric current – when run this way, a motor becomes a generator.

So a car that uses an electric motor to drive the wheels can be used as a motor as you speed up, and a generator as you slow down.

The ability to recover energy from the brakes is where the big savings can be made, especially in town driving, and especially for heavier vehicles. A 2000kg car going at 40 mph has the kinetic energy equal to about 10ml (2 teaspoonfulls) of petrol, and since internal combustion engines are only about 20% efficient, that’s really about 50ml worth – which you burn every time you brake.

And that ‘s why someone came up with regenerative braking.

Aside on electric cars: Electric motors are generally quieter than standard engines. A very real challenge, believe it or not, for electric and hybrid cars, is pedestrian safety. They’re having to test simulated engine noise generators! Strange but true.

Coming soon?

Why not Solar Power?

Many people have suggested that solar panels, aka photovoltaic (PV) cells, may have utility for electric and hybrid cars. Surely it would help to add some panels on the roof and bonnet?

It sounds good, but alas, solar power is fairly dilute – about 1kW per square meter is an estimate used in the industry. And while a few kW may drive your household’s minor appliances, it doesn’t do much for a car. Even small car engines pack over 50kW. There is simply not enough return, even if solar panels were 100% efficient.

When you add the fact that the real “insolation” (power of sunlight) is about 1/10th of the industry claimed 1kW/sqm, and that solar panels are typically only 20% efficient you can see where this is going.

Energy storage

We have already noted that energy storage is key to the challenge. Petrol and diesel pack a punch, but are not renewable and belch carbon dioxide, while batteries would need to be enormous and take ages to recharge.

Nickel Metal Hydride Battery

Hydrogen is not dense enough so you need a colossal reinforced tank if you want to go any distance, and pocket-sized cold fusion reactors are still firmly in the SF domain.

There are however a profusion of ways to store energy, so what other options do we have?

Motorsport fans may have heard about the new F1 regenerative braking systems, called KERS (Kinetic Energy Recovery System), will also know that they are looking at flywheels and capacitors as alternatives to batteries.

A flywheel

The flywheel is simply a spinning disc which stores the kinetic energy in rotational form. This sounds simple, and can be very efficient but requires a fantastically strong flywheel, and will disintegrate if over-charged. Flywheels also have an annoying habit of resisting turns – that is to say their axis of rotation likes to stay pointing the same way, and so would probably need to be mounted within gimbals to allow a car to go around corners…

Flywheels can however achieve pretty good energy densities (comparable to batteries) and are very efficient. They can charge up and down faster and more often than batteries, but are, on the downside, far more expensive.

Ultracapacitors from Maxwell

Capacitors are much cheaper – and can also charge and discharge quickly and many times. The capacitor is the electrical equivalent of a spring. You apply the electric equivalent of pressure (voltage) to the capacitor and it creates an electric field which stores energy. It is fundamentally different to a battery in which an applied voltage supplies electrons which drive reversible chemical reactions; it can charge and discharge much quicker too.

So what’s the catch? Rubbish energy density. Significantly worse than batteries or flywheels (which are of course already pretty poor compared with hydrocarbon fuels).

There are many other clever ways to store energy – in springs, elastic bands, compressed gases, raised weights – plenty for inventors to think about, but, alas, too many to cover here.

Despite this profusion, neither batteries or any of the alternatives pack the punch of petrol, so it’s going to figure in our future.

In my next article I ask: what we can do to make the most of our friend, the internal combustion engine? I look at their efficiency and also take a detour of perhaps a ‘green’ way to keep using them: with biofuels.

================

Series home…

Q: why are some cars so much more efficient than others?

Many people will say small=good, big=bad.

That’s a pretty good start, because big cars are probably heavier and also have more wind resistance.

But the rule does not work universally. Sports cars may be small – and look aerodynamic, but may still use lots of fuel so what’s going on there?

The sad reason for that is that the cars and trucks we drive have most often been optimised for speed, power and appearance and not necessarily for efficiency. This may be due to the circumstances in the 20th century – in which the vehicles we use today evolved – where the supply of oil appeared more-or-less infinite and where there were apparently no consequences for burning it.

So what factors determine how efficient a car or truck will be?

#1 Engine type

Petrol or diesel?

There are two major competing technologies – petrol and diesel. 

Diesel engines were not promoted for many years (for cars) as they tend to be sluggish and noisy. However they are the more efficient technology in terms of miles per gallon and carbon dioxide emissions (CO2 is the main ‘greenhouse gas‘).

However they have become steadily smoother and more refined and really took off in the 80’s – helped somewhat by government incentives (mainly in mainland Europe). However, they soon became implicated in city smog problems, so many incentives were dropped.

The engines continue to improve with efficiencies now perhaps 20% better than the equivalent petrol version – and as concern over CO2 grows diesel engines are again increasing popularity.

So why are diesel engines more efficient?

This is for two reasons: firstly, the engine internal losses are lower; petrol engines control their speed by ‘throttling’ the air supply to the engine – that is to say they hold back the air and make the engine suck it in through a small gap – a tactic which takes a lot of energy. Diesel engines just let the air in with arms wide open.

Secondly, diesel engines have a higher compression ratio which allows them to extract more energy (according to the laws of thermodynamics). Petrol engines cannot compress their fuel any more than they already do because petrol will spontaneously explode if compressed too much, and push the compressing piston backwards. It was this very problem that led Rudolf Diesel to invent the Diesel engine in the first place. In his design the fuel is only added after the air is compressed, totally side-stepping the issue.

#2 Car Dimensions

The most important measurement is the frontal projected area; this is unfortunate because a big cross section is what makes a nice roomy car. Tall cars are bad, wide cars are bad. It’s best to use the length if you need volume – which is the trick trains use.

It also turns out that nice fat tyres are also bad. Sorry. Biscuit wheels have less rolling resistance.

Sports cars do not slip through the air, they push it upwards...

What about aerodynamics though? Surely a Ferrari slips through the air? Yes, to a degree, but not as much as you might hope – for one thing, Ferraris are surprisingly wide, but more interestingly, they are not actually designed to slip through the air – they are in fact designed to push the air upward in order to get downforce. Those big wings at the back are not there to cut through the air – they’re there to give the tyres more grip…

#3 Car Mass

It is hard work to lug a heavy suitcase around. It is obviously hard work to lug a heavy car around. That said, the laws of physics say that heavy items also have more momentum and so will roll further once moving – so you can in fact recover much of the energy if you take advantage of that momentum. The energy is really only ‘lost’ when you use the brakes and turn it to heat. 

This energy recovery trick, combined with ‘regenerative braking’ can reduce the impact of car weight, however generally speaking all additional weight is bad.

PS. I will cover efficient driving tactics and also ways to recover energy from the brakes in forthcoming articles in this series.

#4 Transmission

I know automatic gearboxes are nice. They free up the brain to worry about other things. But they are still bad for fuel efficiency – and for some interesting reasons.

Firstly, fuel efficiency depends on always being in the right gear to get the most out of the engine. However even the best and newest models still don’t change gear at the ideal time – whereas a skilled driver could.  Why not? Surely the brainy chaps at Toyota have perfected it all by now? Alas, no. The reason is this: the gearbox doesn’t have all the information the driver has. The driver may know he or she is about to crest a hill or hit some traffic. The gearbox can’t know this – it simply has to make a judgement based on the angle of your foot on the pedal and the current applied torque.

Secondly, some automatic gearboxes are less good at ‘coasting’ (i.e disconnecting the wheels from any internal machinery to reduce friction and engine resistance).

Aside for health and safety officials from the government: Some might say coasting is unwise; it is suggested that having the power to both slow down or speed up a short notice may improve safety; it is also suggested that engine braking is safer than foot braking because it is less inclined to the lock the wheels. These arguments do actually hold some water. While it is in fact possible to use engine braking to put the car into a skid, it is, in general, wise to take corners in gear as you will have more precise control over speed, and in the (rare) case of brake failure (or the more likely case of a foot slipping off the brake pedal), control is more likely to be maintained. I suggest therefore that coasting should only be done in safe conditions and at low speeds.  

The last problem with automatic gearboxes is all the fluid. While manual gearboxes do have some oil to churn,  the average automatic gearbox is a veritable fountain. The engine connects to the gearbox through a torque converter, which is a cunning clutch-like device based on discs spinning in fluids.

In the gearbox itself, the rotating shafts are used to pump fluids at high pressures through various tight channels and orifices in order to drive actuators that engage and disengage the absurdly clever planetary gearing. All this fluid-flow has inherent frictional loss, and thus reduces efficiency.

#5 Efficiency of Utilisation

What do I mean by ‘utilisation’? Well answer me this: what is more efficient, a Toyota Prius or a Grand Voyager? 

The answer is clear: it depends  – on how many people you are carrying.

Much can be said for using the vehicle right for the job. If you need a big van once every six months, look at renting or borrowing one, and drive something smaller.

#6 Age and Embodied carbon

This time I am asking: what is more efficient, a nice new Prius or your 3 year old one? You might think a new one would have the edge, but what you need to remember is that it takes an awful lot of carbon to make a new car, and it may take years to work that off. Rather keep your three-year-old in tip-top shape and hold onto it until its efficiency starts to dip –  until the new model really is significantly better.

Classic Cortina

If however you don’t have a Prius but have an old guzzler, the carbon may balance out rather more quickly – it really won’t do to keep driving your 3-litre Cortina for ever.

#7 On-board computer

If you get a car with an instantaneous fuel consumption display, you may find yourself trying to set a record – and learning a lot on the way. 

———————

In my next article I discuss some of the innovative green car technology that is on the horizon.

============

Series home…

The 20th century has the seen the rise of the car in the western world. It has impacted the evolution of our cities, our roads and our houses – and we have therefore become rather dependent on them. We co-exist!

However, the effects of carbon emissions are emerging now and we find ourselves on the horns of a dilemma – to drive, or not to drive?

For those of us who can’t give it up, I have written a series of articles looking at the environmental impact of the motor car and looking at what we should be doing to minimize our impact.

Gas guzzler.

1. What makes an efficient car?

2. Green car technologies explained…

3. Making the internal combustion engine greener

4. How to drive more efficiently

That’s it for now, but I will hopefully add more in time.

©Jarrod R. Hart 2009