Engine size explained

Most petrol and diesel cars come in a range of different engine sizes. Engine size refers to the volume of the cylinders inside an engine, and not how it appears from the outside. The larger the engine size, the more fuel and air can fit inside creating a bigger boom, giving larger engines higher power, but these days engine size is less of a direct indicator of how powerful an engine is.

As a general rule, larger engines use more fuel and emit more emissions than smaller ones. For some time now, manufacturers have been downsizing their engines to smaller units to improve efficiency and fuel economy and adhere to increasingly stringent emissions regulations. Technology such as turbocharging and hybrid drivetrains have played a big role in reducing engine size while retaining a good balance of performance. Today, a car fitted with a 1.0-litre engine can be just as powerful as an older car with an engine twice that size, but lower fuel consumption and decreased emissions should help keep your car’s running costs down.

What does engine size mean?

Engine size, also known as ‘engine capacity’ or ‘engine displacement’ refers to the total volume of the cylinders in the engine – this is usually expressed in litres or cubic centimetres. So a car with a 1,000cc (cubic centimetres) powerplant could also be referred to as 1.0-litre. Often, however, engine sizes tend to be rounded up or down to the nearest tenth of a litre (1,380cc is usually expressed as 1.4 litres, while 1,320cc would be classed as 1.3-litre).

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Essentially, the larger the volume of the cylinder, the more room there is for air and fuel inside it, which dictates how much power it can produce. This was especially true of older engines, but the widespread use of turbocharging in modern engines means that smaller engines are much more potent than they were before. This is because they’re able to force more air – one of the vital ingredients for a more powerful explosion when the air and fuel are ignited – into the engine to produce more power.

Some examples of engines that use this technology to produce big power figures from a small engine size are Ford’s EcoBoost, Suzuki’s BoosterJet and Volkswagen’s TFSI series.

Widespread use of hybrid technology is another way that manufacturers are making petrol and diesel cars more efficient. ‘Mild hybrid’ technology uses small electric motors to take the strain off the engine and assist in power delivery during hard acceleration. Most manufacturers now have their own unique versions of mild-hybrid tech, for example, Suzuki’s SHVS system. Mild hybrids usually consist of a powerful starter motor and generator combination which delivers a small boost of torque at low engine speeds while harvesting energy for the battery that powers it when slowing down. Some manufacturers like Volvo, now include at least this form of electrification on every car sold.

How important is engine size?

While engine size used to be more relevant, a car’s power figure is a much more informative way to gauge its performance than engine capacity alone. In the UK, the most common measurement for a car’s power is brake horse power (bhp) which Carbuyer uses. 

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To confuse things still further, there are various different systems of horsepower measurement, and they aren’t all directly comparable. Many manufacturers quote ‘PS’ (PferdeStarke), which roughly translates as ‘horse strength’ in German, or metric HP (horse power). The latter two don’t take into account frictional losses in the engine, so tend to deliver a slightly higher power figure. For example, a single PS or HP equates to 0.99bhp. Also, the increasing numbers of electric vehicles means some car firms are now rating the power of their internal combustion engines in kW (kiloWatts), with 1kW equivalent to 1.341bhp.

What does two-litre, 2.0-litre, or any other number like 1.5 mean?

Until fairly recently, car model designations often referred to the engine size as well as the trim level. The bigger the number, the more expensive the car usually is to buy.

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If you encounter a number like 2.0, or a phrase like 2.0 litres, this refers to the engine’s capacity. This is the combined capacity of all the engine’s cylinders. Typical modern engines have three, four, six or sometimes eight cylinders – although some have more or fewer – so a 2.0-litre four-cylinder engine will have a capacity of 500cc in each of its cylinders.

Each piston moves up inside its cylinder to force a mixture of air and fuel into the combustion chamber. Here, it's compressed and burnt, the explosive force of which causes each piston to move back down inside its cylinder. It's that momentum which is harnessed as engine power. If a four-cylinder engine is described as a 2.0-litre, this means each piston can compress roughly 500cc of fuel and air into the combustion chamber every revolution the engine turns.

If this engine is running at 3,000rpm, that means that every piston in the engine can burn 500cc of fuel and air 3,000 times a minute. The more air and fuel an engine can burn, the more power it usually produces.

How does engine size affect performance?

As a larger engine is generally able to burn more fuel and produce more power, a car with a larger, more powerful engine is likely to be able to accelerate faster and tow heavier loads than a car with a smaller engine can manage.

This statement is less accurate today than it was in the past, given that smaller turbocharged engines are able to produce more power than some bigger, more old-fashioned engines. However, there is a difference in power delivery in a smaller turbocharged engine compared to one without a turbo (known as naturally aspirated.)

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When you put your foot down on the accelerator in a naturally aspirated car with a bigger engine, you’re likely to feel the acceleration more immediately. In a turbocharged car, there can be a delay known as ‘turbo lag’ – this happens because it can take a few seconds for the turbo to spin and suck air into the engine. However, turbo lag is much less of an issue with smaller turbocharged systems and those used in modern engines

It’s also worth bearing in mind the size of the car the engine will be in, because the heavier the vehicle the more performance and efficiency will be blunted. For example, a 1.4-litre engine in a supermini will usually deliver sprightly acceleration and strong fuel economy, but the same engine in a compact SUV will be required to work much harder to achieve similar performance and will use more fuel in the process.

How does engine size affect fuel economy?

With a larger engine able to burn more fuel with each revolution it turns in a minute (rpm), it’ll usually consume more fuel than a smaller engine would during the same journey.

This is a very important consideration when it comes to choosing a new car. With more powerful, bigger-engined cars usually costing more and using more fuel than those with a smaller engine, it’s worth thinking about how much power you actually need.

If your everyday driving typically doesn’t involve much hard acceleration, carrying of heavy loads or cruising at high speeds, you may find that a smaller, less powerful engine will save you money on fuel. Company car users will make a saving on Benefit-in-Kind (BiK) tax, too, as that's linked directly to CO2 emissions. You can read more about CO2 emissions and fuel economy in our guide.

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Small engines tend to particularly suit cars that are used predominantly around town. They provide enough performance for short journeys - like trips to the supermarket, school and the office - where high speeds and rapid acceleration aren’t really necessary. As the engine isn’t regularly needed to produce lots of power, it makes sense to keep it small and take advantage of the gains in economy.

Larger engines, which don't have to work as hard to produce high levels of power, were formerly the default among those who make frequent high speed motorway journeys. However, modern technology can make a small engine behave like a much larger one, and even a modestly sized engine might be perfectly at ease on a long motorway journey.

Remember that your driving style will also dictate how much fuel you'll use.  Keeping the revs low by changing up to the highest possible gear will help save fuel, as will accelerating and braking gently. Keeping tyres correctly inflated could save you hundreds of pounds each year. Click here for our tips on saving fuel through frugal driving.

Your car's engine size and power will also have an effect on your insurance premium. Cars in low insurance groups (i.e. that are cheap to insure) tend to have smaller, less powerful engines.

What’s the difference between petrol and diesel?

Petrol and diesel are both derived from oil but the way they are produced and the way they are used inside car engines is different, which is why you should never put the wrong fuel in your car. Diesel is more energy rich than petrol per litre and the differences in how diesel engines work make them more efficient than their petrol counterparts.

A diesel engine of the same size as a petrol engine will invariably be more economical. This might make the choice between the two seem straightforward but sadly it isn’t, for several reasons. One is that diesel cars are more expensive, so often you need to be a high-mileage driver in order to see the benefit of the economy versus the higher price. Another related reason is that diesel cars need regular runs on the motorway to stay in good condition, so if you only want a car for town driving, a diesel may not be suitable. A third reason is that diesels produce more local pollutants like nitrous oxide, which have more of an impact on air quality. This can also incur further costs in pollution-controlled areas such as the ULEZ in London.

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Petrol and diesel engines have different characteristics. Diesel is a good fuel for long-distance, low-rev driving, such as motorway cruising. It also produces lots of power at low engine speeds, making it ideal for towing caravans.

Petrol, on the other hand, is often better for smaller cars and is generally more popular in hatchbacks and superminis. In terms of fuel economy, the choice between a diesel and a petrol engine can be tricky – see our 'petrol or diesel' guide here.

Why would I want a large engine?

While small, turbocharged engines can produce more power than many bigger engines made in the past, it still holds as a general rule that a large engine is capable of producing more power. Buyers that would benefit from a large engine include caravan owners and people intending to travel long distances on motorways, particularly if the car is full. Cars with large engines can also be fun for those who enjoy driving, as they tend to deliver extra power and noise – an important ingredient for fans of fast cars.

Additionally, cars that are large and heavy in their own right tend to require larger engines. Posh 4x4s like the Range Rover (which weighs a couple of tons) require more energy to get moving and maintain speed.

It's hard to give an absolute rule on what engine size will be sufficient for your specific needs because there are engines of similar sizes that perform significantly differently. However, most engines produced today that are bigger than 1.0-litre or are turbocharged should be more than capable of coping with motorway drives.

If you don't want an engine in your next car, check out our guide to the best electric cars.

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