Diesel Good, Petrol Bad

What?! Haven't I got that the wrong way round? Everybody's trying to kill the diesel. Well, I believe that our politicians have got things the wrong way round. They need an urgent wakeup call and the truth explaining to them. Here it is.

How they work

First off, it helps to understand a little about how the two types are different. Both fuels are essentially hydrocarbon chains, which the engine burns in air. In a diesel engine the air is first compressed so that it gets hot, then the fuel is sprayed in and spontaneously combusts in the hot air. In a petrol engine the fuel and air are mixed and compressed (in either order), and then an electrical spark is generated to ignite the mixture. Petrol must evaporate easily, so it is made from short, light chains. Diesel fuel must survive high levels of compression heating without degrading (many times higher even than so-called "high-compression" petrol engines), so it is made of longer, heavier and less volatile chains. So petrol burns much more easily than diesel. But those long chains also release more energy when they burn. Weight for weight diesel gives out more horsepower than petrol and is therefore more economical. For any given power output, you have to burn more petrol than diesel.

Emissions from diesel and petrol engines

The key pollutants

If you burn more fuel, then you are going to end up with more carbon dioxide (CO2) emissions. Here, the economical diesel scores over the thirsty petrol engine.

Because the longer chains in diesel are harder to burn, they tend to give off more particles of soot, which we call particulates, than petrol. Even greater compression and heating would help, except for two things. Firstly, the fuel might begin to degrade in the heat before it quite reached the cylinder where all the action takes place. Secondly, the higher temperatures would create more nitrogen oxide (NO) emissions. So a diesel has to strike a balance between particulates and nitrogen emissions. Here, the cooler, spark-ignited petrol scores over diesel.

Which is the greener?

Particulates, such as PM10 or PM2, clog up your lungs and can cause various diseases, including cancer. NO is similarly harmful when inhaled. In busy cities around the globe, between them these two pollutants cause hundreds if not thousands of deaths every year. The NO also contributes to the greenhouse effect and global warming. Once you stop producing these pollutants, they will still hang around for many years, perhaps getting on for a century, before rain washes out the particulates and sunlight breaks up the NO. A hundred years of death. No wonder we want to ban them.

Pollution from petrol and diesel emissions

But wait, what about all that CO2? That is the Number One cause of global warming. And global warming is the Number One enemy of us all. Although CO2 is not the most powerful greenhouse gas, it hangs around far the longest, for perhaps the next two thousand years or so. During that time, its total warming effect is greater than the more powerful ones that soon fade. It is constantly drawing in heat, raising the temperature of the environment steadily hotter and hotter. Every day of every year, for thousands of years. It condemns our children's children's children, for many generations, to a relentlessly warming planet and a steadily dying environment all around them – a slow heat death.

The graphs are only sketches, but they show how the two fuels compare. The particulates and NO in diesel (blue) cause a small spike at the beginning but, even during your lifetime, they soon fall away, leaving the higher levels of CO2 from petrol (red) to dominate the coming centuries and accelerate climate change.

Accumulated harm caused by petrol and diesel emissions

Technology is getting better and better at scrubbing the particulates and NO out of diesel exhaust emissions. However the CO2 from petrol is inherent in the fact of burning carbon fuels, the only way to reduce it is via fuel economy. And the diesel is inherently better at that. It may be the dirtier today, but the pollutants it emits today will ultimately do less damage than petrol. And besides that, even today's gap is slowly closing.

The best stopgap

At present, long-haul transport demands the use of fossil fuels. It will be a long time before green electricity or green hydrogen can take their place. Today's long-haul vehicles are at their least efficient and most polluting on short-range and urban stop-go routes, which are usually in densely populated areas. So for short distances electric power really pays. This is why hybrids are becoming so popular. But does it matter whether it is a diesel or petrol hybrid? Yes, very much so!

Diesel engines are at their most efficient when working their hardest, pulling flat-out for long periods. By contrast petrol engines are at their best somewhere in the middle of their working range; they can be more efficient than diesels on a stop-go journey, but are significantly less so when chugging steadily along. In a hybrid, the engine is used to charge the batteries, and batteries like this to be done at a constant rate (it keeps the peak stresses low and prolongs their life). So the engine is either running steadily or shut down. This is the perfect regime to make maximum use of the diesel's good points. Rather than have a large and inefficient petrol engine tuned out of its comfort zone, it is better to have a small diesel running in its most efficient zone. It's something of a no-brainer. But what are we doing, but trying to foist petrol hybrids on everybody? Totally the wrong policy!

The polluter pays

Now I don't know about you, but I believe in the principle that the polluter should pay. This is the crunch, to which everything has been leading. We feel that way over foul discharges, fly tipping, oil spills and all the rest, why not vehicle emissions too? Clear as day, we are the polluters, we should be paying. We should be all-out to curb climate change, whatever the cost to ourselves. We should certainly not be foisting the penalty of our polluting ways onto our children's children's children. Yet that is exactly what everybody is trying to do! We are banning the wrong technology!

We should reverse the fuel premium on diesel and move it to petrol tomorrow. We should progressively ban petrol and petrol hybrids the way we are currently treating diesels, and in a few years time insist on diesel-hybrid only until electric vehicles become practical. Meanwhile we should also work on improving our diesel pollution scrubbers. And we should take the hit we deserve.

Updated 21 Nov 2021