Could the internal combustion engine be greener?

For most people, fully electric cars are some way off. Most people still prefer to have a range of 300+ miles, and to fill up in a few minutes at one of several thousand filling stations.

So how bad is the car we know and love? Can it be made any better? In this article I look the most popular technologies and developments in this area.

The Internal Combustion Engine…

Until all-electric / fuel-cell / nuclear cars are fully realised – we will be stuck with the internal combustion engine. It is therefore well worth looking at how we can make the most of them.

The basics are simple – you heat up some gas, it expands and pushes a piston. In theory you should be able to get all of that heat and turn it into force.

The Internal Combustion Engine

The Internal Combustion Engine

Alas, there are details. We can’t extract all the energy because of the laws of thermodynamics – in fact the limit is about 37% given the temperature range in a typical petrol engine. Then you also have to spend some energy on sucking the gases in. You have to spend energy pumping the gases out. There is friction. There is the heat. The list goes on. Eventually we extract a pathetic 20-25%.

It is therefore not surprising that the piston-driven engine is still being improved despite nigh-on 300 years passing since the earliest versions.  There are still redesigns of the inlet manifold, of the valves, of the crankshaft – all designed to lessen the waste.

To pick one example, we can look at the Atkinson Cycle. One of the many great minds to apply itself to engine efficiency was James Atkinson. He realised that if the power and exhaust strokes were designed to be longer than the intake and compression strokes (see a good animation here), you could let the hot gases expand to a greater volume then they previously occupied – and thus be cooler – so your exhaust gases carry away less embodied energy.

It really works – but is not used. Why? Well mainly because we are too committed to the current design and too much money has been spent in its evolution (which was driven by the need for power, not efficiency); to go back the drawing board has simply been too much hassle. Of course, times they are a-changing…

Another technology waiting for its time in the sun is the “VCR” engine, which stands for Variable Compression Ratio. This is a system in which the volume of the combustion chamber may be adjusted depending on whether you are accelerating, coasting or pulling a caravan up the Col du Tourmalet. It promises to constantly optimise the energy extraction. If its developers can convince major car manufacturers to trust the rather complex crank arrangement it may well be a viable alternative in as soon as 5 years from now. Watch this space.

The Transmission

Enough about the engine – what about the clutch and gearbox? That’s the “tranny” to my American friends, which means something totally different here in the UK.

The transmission’s job is to allow the car to vary speed from 0 to over 100 mph whilst the engine only varies only from around 800 – 6000 rpm, which is a much smaller range. With only 4 or 5 gears you need to use a fair range of engine revs to drive, however, the engine is not equally efficient at all speeds.

Pricnple of the CVT, with thanks to

Principle of the pulley-based CVT, with thanks to the brilliant

The idea has therefore been brewing for a device that can allow complete freedom for the engine to run at its most efficient speed, regardless of the car’s speed.

Its called the ‘continuously variable transmission’ or CVT. The most practical design is the pulley-type (see image), though others are also being developed.

In addition to allowing the engine to run at its most efficient speed,  there is also  no disruption of power flow due to gear changes. All of this will add a few vital % to your overall efficiency.

Aside: The CVT is also a vital part of an electric car using regenerative braking, as the gearing can be used to control the braking effect created by the motor/generator.

The fuel…

While hydrogen produced from renewable electricity (say hydroelectric) is one route to reduced carbon emissions, another is the idea of renewable versions of liquid fuels, the so-called bio-fuels.

While bio-fuels do not have as many issues to overcome as hydrogen, they are still far from a clear-cut solution.

Bio-fuels are simply flammable liquids made from plants (nature’s solar panels) – not only is it a renewable energy source (i.e will not run out), but the growing plants also suck CO2 from the air, so could indeed evolve less net CO2 than sources like coal-derived electricity (or hydrogen made from coal-derived electricity). On the other hand, the balance only works well if the bio-fuel is farmed in an energy (and carbon) efficient way.

The idea is fundamentally good, but as with almost all of the other ideas I have discussed, there is a flip-side.

For the first time, there is a risk of direct competition between poor farmers in remote tropical zones, and the big, fat westerner in his or her gas-guzzler. The latter wants something in their fuel tank and the former wants something in their stomach.

There was much speculation last year that the trend towards bio-fuels was responsible for the crisis in commodity food prices. This may or may not have been the cause – it may well have been the result of irresponsible speculation by commodity traders because few bio-fuel crops have directly displaced food crops. However, the question remains: will the drive for bio-fuels interfere with food supply in the future?

There is also the question of whether rain-forests may be razed to make way for the required crops (there are many choices depending on the rainfall and sunshine levels – cane, corn, beet, sorghum, rapeseed, sunflowers or palms to name a few). If an existing forest is razed it not only destroys biodiversity, but usually also results is a massive belch of CO2 into the atmosphere that will takes many years to offset.

However, bio-fuels are still too promising to let alone.

In Europe most diesel already contains a certain percentage of bio-fuel and the EU has targets for bio-fuel use in transport (5.75% by 2010). Even though the target is unlikely to be met, the trends are strong.

The technical barriers are not too serious – most diesel cars can take bio-diesel blends and can usually be made to take pure bio-diesel with minor adjustments. Bio-ethanol can equally be blended with petrol, and at 10%, many modern cars would in fact run better. However, higher levels are still the reserve of so-called “flex-fuel vehicles’ (FFV’s). These are very popular in Brazil, the world’s leading producer or sugar-based bio-ethanol. The US (especially California) is also leading in this initiative. Although to be fair, this too is not a new idea: the Model-T Ford was a FFV!

In the next article I will looking at the driving techniques that can get 20% more miles out of each tank.


Series home..



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