Petrol or diesel, which engine will serve you best?

fuel station.

Petrol and diesel pumps at a fuel station. An attendant has died in Homa Bay after being hit on the head by thugs who posed as customers.

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I am planning to buy my first car, what’s the difference between a diesel engine and a gas engine? What about the pros and cons, and with this in mind, which should I go for?

fuel station

A pump attendant fuels a car.

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If you are an American, when you say “gas” you mean “petrol”, whose technical name is “gasoline”. If you use English in an anglo way, when you say “gas” in this context you probably mean LPG (liquid Petroleum Gas), which is the stuff you get in cylinders and attach to a stove for cooking, but which can also be used (with remarkably little modification) to power a car.

I am guessing you are using the word gas in the American way, so first: Howdy, stranger!

Let’s start with the difference between diesel and petrol, both of which are “fractions” of crude oil.

Crude oil is a complex mixture of hydrocarbons which refining separates into six main types of product. In very general terms

1. Tops, or gases, such as LPG

2. Spirits, such as petrol

3. Distillates, such as kerosene (technically DPK and colloquially paraffin) and diesels (auto and industrial)

4. Fuel oils, for things like ships and power plants/factories

5. Base oils, mainly for lubricants

6. Residues, like bitumen for tarmac and sealants etc.

Those distinctions may not be hailed as the purest science for research laboratories; they are intended to be indicative to the general public.

The principal difference between diesel and petrol engines (other than the fuel they use) is their ignition.

Spark ignition: Petrol engines suck or inject a mixture of air and petrol (aka gasoline or motor spirit) into the cylinders, squash the mixture into the combustion chamber (a compression ratio of about 10:1) and ignite it with a spark. Bang! That releases the energy of the fuel which is then harnessed as the engine’s power. The engine is turned “off” by cutting the electricity supply that provides a spark.

Compression ignition: Diesel engines inject a mixture of air and diesel (technically “gasoil”, a distillate), and compress it to about double the pressure of a petrol engine (circa 20:1) That is such a squeeze that, even without a spark, it goes Bang! Fuel energy released, engine powered. The engine is turned off by cutting the fuel supply.

In all other methods of function they are much the same.

Each type of system has its pros and cons. The differences used to be quite stark, but have been progressively “levelled up” by advanced technology and design.

Traditionally, and still residually, petrol engines were relatively lighter, a bit cheaper, speeded up more flexibly to higher revs, delivered more horsepower (accelerative force, like a sprinter), were quieter, and were a bit less fussy about the purity of their fuel.

Diesel engines, while coming second in all those respects, delivered more torque power (working force, like a wrestler or a weightlifter) and they were intrinsically more fuel efficient – especially at bigger engine sizes on things like trucks, buses, and agricultural and construction equipment, and in static chug-chug-chug engines used on big generators. Being lower revving and more robustly built to cope with their compression ratio and heavy torque levels they lasted longer. And not having a complex electric ignition system of coils, distributors and spark plugs, they were and are less prone to spluttering in wet conditions. Indeed, given a watertight snorkel on the air intake and exhaust tailpipe, they can (presumably) keep running even when completely submerged.

At this stage of technical development, almost all cars and light utility vehicles were petrol-engine, and almost all big commercial vehicles used diesel engines. Entirely rational. It was almost infradig to drive a diesel-engine car, unless it was a taxi.

This clear dividing line has changed, perhaps led by large 4WD vehicles, for which diesel engine characteristics were more suitable…except for their lack of acceleration and speed in modern usage as “luxury cars”. Enter the turbocharger which solved that downside while maintaining a dramatic advantage in fuel economy over petrol equivalents amid rapidly rising fuel prices. Modern turbo diesel 4WDs use much (!) less fuel than petrol-engine 4WDs delivering the same power and pace. The difference is most significant in bigger engines (circa above 3 litres).

The legendary 3.5 litre V8 petrol engine of the Range-Rover, which was the first mass produced 4WD that could do 160 kph (“100 miles per hour”) and drive over rocks the size of armchairs, is no longer out on its own. There are now numerous 4WDs of many makes that can go even faster and have equal or better off-road ability. Including the latest petrol and diesel-powered Range Rovers.

The technology and trends that started with 4WDs then spread down the size-chain. Fuel price rises made diesel efficiency more interesting, diesel engine longevity was a plus, and combined with quieter engines, easier starting, more flexible engine revs and optional turbocharging, the downsides were neutralised (and even reversed in some respects).

That trend has hit a bit of a speed bump, first through concerns that diesel engine exhausts emit more particulate matter and then in the overall swing from fossil fuel engines of any kind towards electric and hybrid engine power systems. More about those facts and myths some other time.

Meanwhile what car you buy must always start and end with your budget. Whether a petrol or a diesel engine is more suitable for you depends primarily on the size and type of car you drive, whether it is brand new or out-of-date, and on what you use it for, where, and how.

There is no absolute ‘right’ tyre pressure…

car tyre

A man inflates a car tyre.

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 How important is the tyre pressure recommended by car makers, and how exactly should their figure be followed?

The “right” tyre pressure is important – but it does not have to be absolutely exact. And, indeed, it should sometimes be deliberately varied.

The manufacturer’s recommended pressure is not a fixed optimum. It is the best compromise to cover a wide range of normal driving circumstances –road surfaces, loads, speeds, climatic temperatures and journey lengths that are different…but not too extreme. It is also based on the precise tyre specification that is fitted to the car in the factory, and that might not be what is fitted to your car today (most motorists run used cars).

Think of the recommended pressure as your best guide. Set your pressures close to that and you will have a safe setting that gives a good balance of shape, strength and comfort.

The pressure you set should be hard enough to give the tyre its correct shape so the tread lies flat (left to right) on the road surface when it is rotating at speed. But no so hard that it compromises ride comfort and subjects the vehicle to undue shocks and vibration.

Too hard and the centre of the tread will bulge slightly, causing rapid wear in the middle and lifting the edges of the tread off the road so they do not contribute to traction.

Too soft and the edges of the tread will take the load, fractionally lifting the centre of the tread off the road. It will also allow the sidewalls to bulge more, so they flex more and generate more heat, and are also more vulnerable to damage from roads made of sharp stones.

So, first consider whether the specification of tyres on your car today are much the same as, or very different from, those originally fitted by the manufacturer. If roughly the same…

Next, for all “normal” conditions set and keep the pressures at or very near the manufacturer’s recommended level(s).

Generally, err on the high side by a few per cent (a couple of psi) as your norm rather than under-inflate.

The only situation in which you should deliberately under-inflate your tyres is when driving in deep, soft sand. Do not deflate until you reach the sand and re-inflate as soon as you leave it. Cars driven over desert dunes drop their pressures by more than half…the tyre then “floats” better on the top of the sand instead of digging into it like a circular saw and getting stuck. This technique is combined with a lower gear and higher revs to ensure there is enough power to maintain a good momentum.

Significant departure from the recommended pressure should be in exceptional circumstances for extreme journey conditions, and the change will almost always be to increase the pressure for a whole journey, by 5-10% for each of the following conditions:

Speed: Sustained high-speed cruising over long distances (to reduce heat build up by making the sidewall stiffer so it flexes less. Flex is a heat generator).

Load: If the vehicle is heavily or fully laden, so the tyre retains its normal shape.

Road surface: On road surfaces made of jagged stones, there is a higher risk of gashing the tyre wall if it protrudes well outside the tread. There is also greater risk of a “pinch” puncture if an unseen rock squashes the sidewall flat against the wheel rim. Higher pressure to stiffen the sidewall reduces both these hazards. A hard tyre is also less prone to thorns and nails through the tread.

Remember to reset the pressures when the journey ends. Delaying this will not do any immediate harm, but it will make the ride less comfortable and in the longer term could cause body cracks and shorten the lifespan of rubber bushes.

Why cars start to ‘use’ oil

Engine oil

A mechanic adds engine oil.

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Dear Gavin, Thanks for your advice on motor vehicle issues.  I have Subaru Outback 2008 with a BP9 engine. I have recently needed to add engine oil several times between services while previously I only added at service time. What could be the problem?

As engines approach an advanced state of wear they start to “use” oil (usually by seepage past the piston rings into the combustion chamber) so topping up becomes necessary between services. Initially, this is only a small quantity added at under-bonnet checks to keep the oil level at the Max line. The quantity of oil that needs to be added will progressively increase as the wear continues.

It is generally accepted that if the total topping up required between services is not more than half-a-litre, the engine is still competent and serviceable as it is. If more than half-a-litre is needed, the rate of wear will only get worse from here on, the exhaust will start to emit light blue-grey smoke when revved hard, engine performance will decline and fuel consumption will go up. There will be a higher risk of consequential damage in addition to the existing wear. A car that emits gouts of blue-grey smoke all the time should have had an engine overhaul long ago.

Delaying repair will not save money – not in running costs nor the final repair bill. You should confirm that it is overall wear (not some other cause of oil loss) by checking the colour of your exhaust fumes at high revs, and/or with a compression test which is a quick and simple procedure.

The remedy is usually at least a replacement of the piston rings (if done promptly the pistons and cylinder liners might still be okay; if delayed they can get more expensively worn or scratch-damaged. Piston rings could break and cause even more trouble.

Because the engine needs to be dismantled to replace the rings, the opportunity is often taken to replace any other well-worn parts at the same time (especially bearings), and to clean up and re-seat the valves. It is a labour-intensive exercise, it means replacement of the cylinder-head gaskets when the engine is reassembled, but it is not a major cost and it will give your engine a new lease of life. You will be saving money instead of wasting it as you continue motoring.

Coming soon: When brake fluid and coolant need to be changed, demystifying drive configurations, dismantling engines, and more.

Do you have a motoring question, email:  [email protected]

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