The engine: petrol, diesel, and the drive to the wheels

You can drive for twenty years without knowing what happens inside an engine, and plenty of people do. But three of the jobs this course gives you only make sense once you know. The coolant check in lesson 9 exists because of what an engine does with its fuel, and most of what it does isn't moving the car; the oil check is the harder case, and this lesson says why. The cambelt (the timing belt, in American English) is replaced by the calendar and the mileage, not only when it starts to complain, because it often doesn't. And a diesel's glow-plug light asks you to wait before you start, for a reason that comes straight out of how a diesel lights its fuel. This lesson is how a petrol (gasoline) or diesel engine turns fuel into motion, where most of that fuel's energy goes instead, what keeps the engine's parts in step, and the four kinds of gearbox (transmission) that carry the result to the wheels.

Nothing in this lesson asks you to open the bonnet (the hood). Its exercise is done at a table, with the car's handbook (the owner's manual) and whatever service record the car has.

Four strokes

The US Department of Energy's vehicle research office (DOE) puts the whole engine in a few sentences on its page "Internal Combustion Engine Basics", and they're worth reading slowly. Combustion is burning, "the basic chemical process of releasing energy from a fuel and air mixture", and in an internal combustion engine it happens inside the engine itself. Then: "The engine consists of a fixed cylinder and a moving piston. The expanding combustion gases push the piston, which in turn rotates the crankshaft. Ultimately, through a system of gears in the powertrain, this motion drives the vehicle's wheels."¹

That is the chain. Burning gas pushes a piston, the piston turns a crankshaft, and gears take the turning to the wheels. Most of this lesson is about what feeds that chain, what keeps it in step, and what it leaves behind, which turns out to be mostly heat.

Most car engines work in four strokes. DOE again: "Most of these are four-stroke cycle engines, meaning four piston strokes are needed to complete a cycle. The cycle includes four distinct processes: intake, compression, combustion and power stroke, and exhaust."¹ What happens in each depends on the fuel, and the schematic below sets petrol and diesel side by side.

Schematic, drawn by this course from DOE's four named strokes and its petrol and diesel sentences, and AFDC's glossary for the exhaust system.¹²³ It shows what the sources say happens in each stroke, not how the piston moves. A petrol engine that injects its fuel straight into the combustion chamber (AFDC's term) isn't shown at intake, because no source read says at which stroke that fuel goes in.

Notice what the sources give and what they leave out. They say what comes in, that the piston compresses it, what lights it, and that the expanding gas does the work. For the fourth stroke, DOE gives only its name, and AFDC's glossary says the exhaust system "channels the exhaust gases from the engine out through the tailpipe."² How the piston travels in each stroke, and how the gas gets in and out at the right moments, aren't described in anything this course read beyond one line you'll meet in the cambelt section, so this lesson doesn't draw them. The textbooks in Go deeper have chapters on engines; this course read only their contents pages.

Spark or squeeze: petrol and diesel

DOE says there are "two kinds of internal combustion engines currently in production: the spark ignition gasoline engine and the compression ignition diesel engine."¹ The names tell you the difference, once you know what each is lit by.

Petrol

For a petrol engine, DOE writes: "In a spark ignition engine, the fuel is mixed with air and then inducted into the cylinder during the intake process. After the piston compresses the fuel-air mixture, the spark ignites it, causing combustion."¹

That's true of some petrol engines and not others. The Alternative Fuels Data Center (AFDC), another US Department of Energy site, says the fuel "is injected into either the intake manifold or the combustion chamber, where it is combined with air, and the air/fuel mixture is ignited by the spark from a spark plug."² AFDC doesn't define the intake manifold; on this course's reading, what matters here is only that it's before the combustion chamber, so fuel injected there meets the air on the way in. So a petrol engine may mix its fuel with the air before it reaches the combustion chamber, in the intake manifold, which is the case DOE's sentence describes, or inject it straight into the chamber. Either way, the spark does the lighting.

Diesel

A diesel works the other way round. DOE: "In a diesel engine, only air is inducted into the engine and then compressed. Diesel engines then spray the fuel into the hot compressed air at a suitable, measured rate, causing it to ignite."¹ AFDC's account of how diesel vehicles work says the fuel is "ignited by the high temperatures achieved when the gas is compressed by the engine piston."³ The compression is the lighter. That is why it's called compression ignition, and why a diesel has no spark plugs.

fueleconomy.gov, the US government's fuel-economy site, adds two figures about diesels. It says "diesel fuel contains roughly 10% to 15% more energy than gasoline. So, diesel vehicles can often go about 20% to 35% farther on a gallon of fuel than their gasoline counterparts", and that "Diesel engines have inherently lower losses and are generally one-third more efficient than their gasoline counterparts."⁴⁵ Those are a US government site's general figures, not a verdict on any car. Whether a diesel suits you is not this course's question; lesson 3, "Diesel exhaust: the DPF and AdBlue", is about what a modern diesel asks of the way it's driven.

Both kinds of engine are run by a computer, which AFDC calls the electronic control module (ECM): "The ECM controls the fuel mixture, ignition timing, and emissions system; monitors the operation of the vehicle; safeguards the engine from abuse; and detects and troubleshoots problems."² The dashboard's warning lights are lesson 11's.

Glow plugs, and a cold morning in Thurso

DENSO, a company that makes glow plugs (so it writes about its own product), explains the problem: "To start combustion within a diesel engine, air is compressed in a cylinder to raise its temperature and pressure, fuel is then added to start the combustion cycle. If the temperature of the air is too low, then some of the fuel that is supplied may not be burnt." Its answer: "glow plugs provide extra heat to the air and ensure that the cylinders reach the required temperature for a clean combustion cycle."⁶

It's a January morning in Thurso, on the north coast of Scotland, and the car on the drive is a diesel that has stood outside all night. (DENSO's light comes on at every start, not only in winter; a cold morning is just when its reason is easiest to see.)

1. The driver switches on the ignition, and a light comes on: the glow-plug warning light. DENSO says it comes on "as the vehicle's ignition is turned on, because this warning is to inform the driver that the glow plugs are going through their heating cycle in order for them to be hot enough to make starting the engine quick and efficient."⁶
2. So the driver waits. DENSO says the driver "should wait until the light goes out, before starting the engine."⁶ The waiting gives the plugs time to heat, and that heat is what helps the cold air reach the temperature the fuel needs.
3. The light goes out, and the driver starts the engine.
4. If the light stays on once the engine is running, that is a different matter. DENSO says the light "should not be ignored if it remains on while the engine is running."⁶ Lesson 11, "Dashboard warning lights", is where lights that stay on belong.

Whose rule is this, and for which car? It's a glow-plug maker's general advice, and your handbook is the authority for yours: it shows what your car's light looks like and what the maker says to do. This course did not read a diesel car's handbook page on the glow-plug light, so DENSO's line is the one quoted here. Waiting for the light is yours. Replacing glow plugs isn't: DENSO says "vehicle manufacturers (VMs) specify changing the glow plugs at specific intervals as part of the vehicle's service schedule", which, on this course's reading, makes it a garage job at the handbook's interval, and one to call someone for.⁶

Where the fuel goes: mostly heat

fueleconomy.gov's page "Where the Energy Goes: Gasoline Vehicles" starts with the number: "Only about 12%–30% of the energy from the fuel you put in a conventional vehicle is used to move it down the road, depending on the drive cycle. The rest of the energy is lost to engine and driveline inefficiencies or used to power accessories."⁴ And it says where the biggest share goes: "In gasoline-powered vehicles, most of the fuel's energy is lost in the engine, primarily as heat. Smaller amounts of energy are lost through engine friction, pumping air into and out of the engine, and combustion inefficiency."⁴

The page's diagrams put numbers on it, for two standard US test cycles: stop-and-go town driving, and a highway run with no stops at an average of about 48 mph (77 km/h, by this course's arithmetic).

fueleconomy.gov, "Where the Energy Goes: Gasoline Vehicles", the labels on its city and highway diagrams, read 26 September 2026. US test cycles (EPA FTP-75 and HWFET). The page's own caution: "Energy use and losses vary from vehicle to vehicle."⁴ Each row is a range, so the rows can't be added to one exact total. The city diagram counts idling inside the engine and pump rows.

The last row is the pumps and systems the engine has to run for itself: "The water pump, fuel pump, oil pump, ignition system, and engine control system use energy generated by the engine."⁴ And even the share that reaches the wheels doesn't all stay as motion. "Any time you use your brakes in a conventional vehicle, energy initially used to overcome inertia and propel the vehicle is lost as heat through friction at the brakes."⁴

So on fueleconomy.gov's figures, most of what a petrol engine does with its fuel is make heat, and moving the car is the smaller part. A diesel loses less, and the same site calls diesels "generally one-third more efficient", but this course found no figure for how much of a diesel's fuel reaches the wheels.⁴

Why that means coolant

All that heat has to go somewhere. The Highway Code, which applies in Great Britain, says in its Annex 6: "Most engines are water-cooled."⁷ Toyota, in the handbook for the 2023 Corolla sold in the US, says what the fluid in that system is for: "Coolant is neither plain water nor straight antifreeze. The correct mixture of water and antifreeze must be used to provide proper lubrication, corrosion protection and cooling."⁸ Toyota names the parts it sits in, too: if the level drops soon after topping up, it tells the owner to "Visually check the radiator, hoses, engine coolant reservoir caps, drain cock and water pump."⁸ That is the water pump from fueleconomy.gov's list, running on the engine's own power.

The sources read don't trace the circuit: how the cooling system gets rid of that heat. None of them explains it, so this lesson doesn't either. What it can tell you, because Toyota says it for that car, is why you never open a hot system: "The cooling system may be under pressure and may spray hot coolant if the cap is removed, causing serious injuries, such as burns."⁸ As lesson 1 showed, the Highway Code's advice (a "should", not a legal MUST) is the same in outline: wait until the engine "has cooled naturally".⁷ Lesson 9, "Under the bonnet: oil, coolant, brake fluid and screenwash", gives the check itself, which is yours, on conditions: engine cold, reservoir read closed, the handbook's coolant only.

What the sources say about oil, and what they do not

Oil is the harder case, because the sources this course read name oil's job and tell you what goes wrong without it, but not how it does that job. This is what they do say.

• It lubricates the engine. The RAC, a motoring organisation that sells breakdown cover and repairs, says of the oil warning light: "If the oil is not lubricating the engine effectively it could lead to expensive or even irreparable engine damage."¹⁰ That's the job named; how lubrication protects the parts is not explained in anything this course read.
• An engine uses oil up. Toyota, in the same 2023 Corolla handbook: "A certain amount of engine oil will be consumed while driving."⁸
• Too little or too much can harm it. Toyota puts "Check the oil level on a regular basis" under the heading "To prevent serious engine damage", and adds, "Avoid overfilling, or the engine could be damaged."⁸
• The engine pumps it round: the "oil pump" is on fueleconomy.gov's list of what the engine drives.⁴
• Friction comes into it. Friction is one of the smaller losses on fueleconomy.gov's energy page, and its maintenance page tells US drivers to look for oil labelled "Energy Conserving" on the API performance symbol, a mark printed on US oil containers, "to be sure it contains friction-reducing additives."⁴⁹
• Low oil is an emergency on the road. The same RAC page says: "If the low engine oil light is on then you should stop as soon as safely possible and switch off the engine."¹⁰ Lesson 11 takes that light, and lesson 16, "When the car stops: where, who stands where, and the call", the "safely".

Those are the consequences. How oil does that job, which is the mechanism, is not in anything this course read, and the lesson won't supply it from general knowledge. The textbooks named in Go deeper have chapters on it. Reading the dipstick is yours, and topping up is yours, on conditions (the handbook's grade, level ground, engine off); both are lesson 9's. This course doesn't teach an oil change.

The cambelt: a date first, and symptoms too

Go back to the four strokes. Something has to let the air in and the gases out at the right point in the cycle, and keep that in step with the piston. The RAC, a motoring organisation that sells breakdown cover and cambelt changes, gives the line this lesson promised earlier: "The camshaft controls the opening and closing of the engine's valves, controlling the air-fuel mixture that enters and is expelled from the engine."¹¹ The valves open and close; a camshaft works them; and the crankshaft is turned by the pistons. Those have to stay in time with each other.

That is the cambelt's job, on the RAC's page "What is a cambelt and how do you replace it?": "The cambelt is a rubber belt that has teeth which are designed to grip onto the cogs on the crankshaft and camshafts, which keeps them in sync."¹¹ "A cambelt controls the timing of a vehicle's internal combustion engines between when the valves to the engine cylinders open and close. This controls the flow of air in and out of the engine."¹¹ The RAC says cambelt is the usual British name, and that elsewhere it's called a timing belt. They are the same part.

The rule: miles or years, whichever comes first

The RAC's range is wide: "Replacement is typically recommended every 60,000 to 100,000 miles, but some vehicle manufacturers may suggest the belt is changed when the vehicle reaches five or six years of age".¹¹ (60,000 to 100,000 miles is about 97,000 to 161,000 km, by this course's arithmetic.) Lower down, the same page's FAQ gives a wider range still, "every 40,000 to 100,000 miles", which is one more reason not to lean on either. That range is the RAC's, across many cars, and it isn't the figure for yours. The RAC itself sends you to the handbook: "The replacement interval for your car's cambelt will be detailed in the owner's manual for the vehicle. This will be after a specific number of years or miles, whichever comes first."¹¹ And: "Regardless of the cost, it's important to have a cambelt change done at the manufacturer's recommended interval."¹¹

So there are three things to know about a car's cambelt, and none of them is how it sounds. What interval does the handbook give, in miles and in years? When was the belt last changed, by date and by mileage? And which of the two limits comes first from that change?

Two more things the RAC says. First, the MOT won't catch it: "At your annual MOT, your mechanic will not check your cambelt."¹¹ (The MOT is Great Britain's periodic test; lesson 21, "The MOT and Northern Ireland's vehicle test", covers what it does and does not look at.) Second, a service might: "if you ask your local mechanic to check the cambelt at the annual service then they should inform you of any problems."¹¹ It doesn't replace the interval, and the RAC's own rule is the interval.

Finding the interval and working out when it's due is yours. Changing the belt is a garage job, so call someone.

A used car in Middlesbrough

Someone in Middlesbrough buys a used petrol car that is seven years old and has done 58,000 miles (about 93,000 km). The seller has no record of a cambelt change. In this example, and only in this example, the handbook's schedule says: replace the cambelt every six years or 60,000 miles, whichever comes first. (The figures are made up for the example. Your handbook's are the ones that count.)

1. Find the handbook's interval. Six years or 60,000 miles.
2. Find the last change. There's no record of one. This course's reading is that a belt with no record of a change has to be treated as the original, fitted when the car was new, because nothing shows otherwise. So the clock started at year 0 and mile 0.
3. Check each limit from that start. Years: seven, against a limit of six. Past it. Miles: 58,000, against 60,000. Not yet.
4. Apply "whichever comes first". The years limit came first, a year ago. The belt is due now, although the mileage says it has 2,000 miles to go.

The trap in this one is the odometer. 58,000 looks comfortably under 60,000, and a buyer who only checks the mileage would think they have time. The years ran out first.

Now one with a wrinkle, for you to do. A car in the US is ten years old and has done 31,000 miles. Its owner's manual (again, the example's figures) says to replace the timing belt every seven years or 90,000 miles, whichever comes first. The service record shows one timing belt change, when the car was four years old and had done 14,000 miles.

What this course does not say about cambelts

If your handbook's schedule has no cambelt or timing belt line, this course can't tell you why: it read no source it can rely on about engines without one, so it says nothing about them. Write "the handbook gives none" and ask at the next service what your engine has; that's this course's suggestion. And the RAC's five signs still matter: its advice is to have a car showing any of them checked "as soon as possible by a qualified mechanic". Their absence isn't a reason to leave a belt past its interval because the car seems fine.

Four ways to the wheels: gearboxes

DOE's chain ended with "a system of gears in the powertrain". AFDC describes the transmission (gearbox, in British English) as the part that "transfers mechanical power from the engine and/or electric traction motor to drive the wheels".¹² Why gears at all? fueleconomy.gov's page on transmission technologies puts it in one sentence: "Adding gears allows your engine to operate at a more efficient speed more often."¹²

You'll meet four kinds.

• A manual, where the driver changes gear, and a conventional automatic, where the car does. Both, in fueleconomy.gov's words, "use a fixed number of metal gears to control the ratio between engine speed and wheel speed."¹² More gears help. Against a four-speed automatic, the page gives about 2% better efficiency for six gears, 2% to 3% for seven and 3% to 4% for eight, on figures from the National Academy of Sciences in 2015.¹²
• A continuously variable transmission (CVT). "Instead of gears, CVTs use a pair of variable-diameter pulleys connected by a belt or chain that can produce an infinite number of engine-to-wheel speed ratios."¹²
• A dual-clutch transmission (DCT). "DCTs operate much like manual transmissions, except that they use two clutches and automatic shifting. They are generally not quite as smooth as regular automatics, but manufacturers are making strides to improve this."¹²

What an electric car has in place of a gearbox is for lesson 4, "Hybrids and electric cars: how they work".

Manual or automatic: two pages, one site

Here the same government site says two things that pull different ways. The transmission page says "Automatic transmissions are less efficient than manuals due to parasitic losses."¹² The site's page of myths answers the claim that manuals always win: "Advances in automatic transmissions have improved their efficiency to the point that the automatic version of a vehicle often gets the same or better fuel economy than the version with a manual transmission."¹³

This course's reading is that, read together, they don't contradict each other. One is about the kind of gearbox; the other is about what you'll actually find when you compare the two versions of one model now. The myths page also says what to do about it: "For vehicles offered in both automatic and manual transmissions, consumers can easily compare fuel economy using our Find a Car feature."¹³ That's a US tool, for US figures. This course did not read a UK source for comparing a model's official figures.

What a gearbox needs from its owner is in the handbook's schedule. This course read no maker's gearbox service line, so it gives none.

What people get wrong

"Every petrol engine mixes its fuel with the air before the cylinder." DOE's sentence describes that kind. AFDC says the fuel is injected "into either the intake manifold or the combustion chamber", so some petrol engines inject it straight into the combustion chamber.¹²

"A diesel's glow plugs light the fuel, like spark plugs." DENSO, which makes them, says they "provide extra heat to the air". The fuel is lit by the heat of compression, which is the difference the name "compression ignition" describes (DOE; AFDC).¹³⁶

"Most of the fuel moves the car." On fueleconomy.gov's figures, 12 to 30 per cent does, and most of the rest is lost in the engine as heat.⁴

"The MOT checks the cambelt." The RAC, which sells breakdown cover and repairs, says your mechanic "will not check your cambelt" at the MOT.¹¹

"A cambelt will warn me before it goes." The RAC, which sells breakdown cover and repairs, gives five signs, says a car showing any of them should be checked "as soon as possible by a qualified mechanic", then adds "Many cambelts show no obvious symptoms before failure."¹¹ That is why the rule is the handbook's interval, whichever comes first.

"An automatic always uses more fuel than a manual." fueleconomy.gov's myths page says the automatic version of a model "often gets the same or better fuel economy".¹³

Practice

For your plan

This lesson adds four lines to the car file you're building. The fuel (petrol or diesel) and, for a diesel, what the handbook says about the glow-plug light, with its page. The cambelt: the handbook's interval in miles and years, the date and mileage of the last change or "no record", and the date or mileage at which the next is due, whichever comes first; or "the handbook gives none", with the question to ask at the next service. And the gearbox type. The cambelt line goes on the file's calendar as a date, because the car won't remind you.

Connections

Lesson 1, "What a car is made of, and why the handbook comes first", made the handbook the authority for every per-car figure, and the cambelt interval is one: the range from the RAC, which sells breakdown cover and repairs, is only a range, and your handbook's two numbers are the ones that count. Its three labels are used here too: waiting for the glow-plug light and finding the interval are yours; the belt change and the glow plugs are jobs to call someone for.

Lesson 3, "Diesel exhaust: the DPF and AdBlue", picks up the diesel from here: what a modern diesel's exhaust has to clean, and what that asks of the way it's driven. Lesson 4, "Hybrids and electric cars: how they work", puts an electric motor beside or in place of the engine, and fueleconomy.gov's energy figures will be worth comparing. Lesson 9, "Under the bonnet: oil, coolant, brake fluid and screenwash", turns this lesson's reasons into checks. Lesson 11, "Dashboard warning lights", takes the oil, temperature and glow-plug lights. Lesson 12, "Fuel economy: what maintenance buys, and the figures that were withdrawn", comes back to fueleconomy.gov and to oil grades. And lesson 24, "Choosing a garage, servicing and warranties", is where the service schedule this lesson's exercise opened becomes a conversation with a garage.

Go deeper

• US Department of Energy, "Internal Combustion Engine Basics". Read here in full. Short, official, and the source of this lesson's four strokes and its petrol and diesel sentences.
• fueleconomy.gov, "Where the Energy Goes: Gasoline Vehicles". Read here in full. Its diagrams let you switch between town and highway driving and see every loss, including braking, which this lesson only mentions, and wind resistance, which it leaves out. The site has matching pages for hybrids and electric cars.
• AFDC, "How Do Gasoline Cars Work?" and "How Do Diesel Vehicles Work?". Read here in full. Each has a labelled drawing of the whole car and a plain glossary of its parts.
• For how oil and coolant actually work inside an engine, the standard textbooks have chapters on them: James Halderman and Curt Ward, Automotive Technology: Principles, Diagnosis, and Service, 7th edition (Pearson, 2025), chapters "Engine Oil and Filters" and "Antifreeze and Coolant"; and Jack Erjavec and Rob Thompson, Automotive Technology: A Systems Approach, 7th edition (Cengage, 2020), chapter 14, "Lubricating and Cooling Systems". This course read only their publishers' tables of contents, so it can tell you those chapters exist, not what they say. Try a library before buying either.

Sources

1. US Department of Energy, Vehicle Technologies Office, "Internal Combustion Engine Basics", page modified 22 June 2023. Read: full article text; re-read 26 September 2026.
2. Alternative Fuels Data Center (US Department of Energy), "How Do Gasoline Cars Work?". Read: full text, including the component glossary; re-read 26 September 2026. No page date shown.
3. Alternative Fuels Data Center, "How Do Diesel Vehicles Work?". Read: full text; re-read 26 September 2026. No page date shown.
4. fueleconomy.gov (US Department of Energy and Environmental Protection Agency), "Where the Energy Goes: Gasoline Vehicles". Read: full body text, with the labels on its city, highway and combined diagrams, 26 September 2026. No page date. The table in this lesson is from the city and highway diagrams; the combined diagram's two labels disagree with each other, so it is not used.
5. fueleconomy.gov, "Diesel Vehicles". Read: full body text; re-read 26 September 2026. No page date.
6. DENSO Europe, "Why might the glow plug warning light come on?". Read: full article text; re-read 26 September 2026. Dated 10 November 2023 on the page (date recorded at lesson 11's Stage 4). A glow-plug maker writing about its own product; its "10 to 15 seconds" rule of thumb and its line on glow plugs and the diesel particulate filter are not used here.
7. The Highway Code (GOV.UK), "Annex 6. Vehicle maintenance, safety and security". Read: the full body of Annex 6, via the GOV.UK content API; last updated 29 January 2022. Applies in England, Scotland and Wales.
8. Toyota Motor Sales USA, 2023 Corolla Owner's Manual (OM02568U), for the US market, section 6-3 "Do-it-yourself maintenance", pp. 305 to 307 (engine oil and coolant), PDF. Read: pp. 305 to 307 in full, 26 September 2026. Toyota's words, for that car.
9. fueleconomy.gov, "Keeping Your Vehicle in Shape", the motor oil paragraph. Read: current page full text, 26 September 2026.
10. RAC Drive, "Dashboard warning lights guide: red, amber and green symbols explained", dated 18 August 2026. Read: full text. A motoring organisation; the page advertises its mobile mechanics and breakdown cover.
11. RAC Drive, "What is a cambelt and how do you replace it?", modified 31 July 2026. Read: full article text; re-read 26 September 2026. A motoring organisation that sells cambelt changes; its price table is not used.
12. fueleconomy.gov, "Advanced Transmission Technologies". Read: full body text, with its table; re-read 26 September 2026. No date; the efficiency figures are the page's, from the National Academy of Sciences, 2015.
13. fueleconomy.gov, "Fuel Economy Myths and Misconceptions". Read: full body text; re-read 26 September 2026. No date.