Throttle bodies and electronic throttles

A petrol engine controls its power by controlling its air, as What happens when you press the accelerator explains. The part that does it is the throttle body: a short metal tube between the air filter and the intake manifold, with a valve inside.

It has only a few parts:

• The bore, the tube the air flows through, typically 50 to 70 mm across on a 2.0-litre engine.
• The throttle plate, a thin disc on a spindle across the bore. Turned edge-on to the air, it’s wide open; turned across the bore, it’s nearly shut. It’s also called the butterfly.
• Return springs that pull the plate shut when nothing is holding it open.
• A position sensor that tells the ECU how far open the plate is.

On older cars, a cable ran from the accelerator to a lever on the spindle, so your foot opened the plate directly. Almost every car now has an electronic throttle, also called drive-by-wire or electronic throttle control. The pedal is a sensor, and a small electric motor turns the plate through a set of gears. BMW was one of the first, on the 7 Series in 1988, and by the late 2000s nearly every new car worked this way.

The demo is the 2.0-litre petrol engine from the earlier lessons, held at 3,000 rpm. The slider is your foot, the graph shows how far the throttle opens for each pedal position, and the buttons choose a drive mode.

1. The pedal has two sensors, so two wires run to the ECU. They measure how far you’ve pressed it.
2. The ECU looks up how far to open the throttle. In Normal, 20% pedal opens it only 12%. Opening gently at first makes the car easy to drive smoothly, because the first few degrees of a throttle let through far more air than the last.
3. The motor turns the plate to that opening, and two position sensors on the spindle tell the ECU where it’s got to. The ECU corrects the motor hundreds of times a second.
4. Behind the plate, the air is thinner, shown by the dots spreading out. The pistons have to suck it in against this partial vacuum, and that costs power: about 3 kW (4 bhp) at 20% pedal, while the engine makes 14 kW (19 bhp). That’s a pumping loss.

Now try the modes at 20% pedal. Eco opens the throttle only 6%, for 5 kW (6.7 bhp), and Sport opens it 30%, for 36 kW (48 bhp). Same foot, very different car. Then floor it in each one: the throttle opens fully every time, and the engine makes the same 58 kW (78 bhp). Sport makes the car feel keener; it doesn’t add power.

Press Sensor fault to see limp mode. When the ECU can’t trust its sensors, it turns the motor off. Two springs, one pulling the plate shut and one pushing it open, meet at a point about 7° open. That’s enough for a fast idle and gentle driving, so you can get off the road, but the pedal does nothing.

The model is simplified, but the numbers match the accelerator and air lessons.

With a cable, the throttle went wherever your foot put it. With an electronic throttle, the ECU treats the pedal as a request for torque and decides for itself how to deliver it. That lets one throttle do many jobs that used to need extra parts:

• Idling. The ECU opens the plate just enough to hold the idle speed steady, and a little more when the air conditioning or a cold engine needs it. Cable cars needed a separate idle air valve.
• Smoothing. Stamp on the pedal and the ECU opens the throttle a fraction more slowly, so the car doesn’t jerk. Lift off and it closes it gently.
• Traction and stability control. When a wheel spins or the car starts to slide, the ECU can close the throttle while your foot is still down.
• Cruise control and speed limiters. The ECU just works the throttle itself.
• Gear changes. Automatic and dual-clutch gearboxes ask the ECU to dip the torque for a moment during each change, and some manuals blip the throttle on downshifts to match the revs.
• Turbo engines. The ECU uses the throttle together with the wastegate to control boost.

Two of everything. A throttle that opened on its own would be dangerous, so electronic throttles are built to fail safe. The pedal has two sensors and the throttle body has two, and each pair reads in a different way, such as one rising while the other falls. The ECU checks them against each other all the time, and a separate part of the ECU checks that the main part is behaving. If anything disagrees, it goes into limp mode. Most modern cars also ignore the accelerator when you press the brake at the same time.

A part-shut throttle doesn’t just limit the air; it makes the engine work to suck air past it. At light loads, when the throttle is nearly shut, these pumping losses are a big part of why petrol engines are so much less efficient than they are flat out.

• Lift off and the pumping loss helps slow the car. It’s a big part of engine braking.
• Diesels don’t use the throttle to control their power. They let in plenty of air and vary the fuel instead, which is one reason they’re so economical on light loads. Many modern diesels still have a throttle valve, but only to help with emissions systems and to shut the engine down smoothly.
• Variable valve timing and lift can let the engine take in less air without closing the throttle as far. BMW’s Valvetronic goes furthest: it controls the air by how far the intake valves open, and leaves the throttle almost wide open. Variable valve timing explains how cams can change.

• A dirty throttle body. The engine’s crankcase breather feeds oily vapour into the intake, and over time it leaves a sticky black coating round the edge of the plate and the bore. Because the plate is nearly shut at idle, even a thin layer matters: the idle can become rough or hunt up and down, and the engine may stall when you stop. Cleaning the throttle body with throttle body cleaner usually cures it.
• A relearn after cleaning or fitting a new one. The ECU learns exactly where the plate sits when shut. After cleaning, disconnecting the battery or fitting a new throttle body, many cars need a relearn: sometimes just leaving the ignition on for a short time, sometimes a scan tool.
• Sensor faults. If the two pedal sensors or the two throttle sensors disagree, the ECU stores a fault code, such as P2135 for two sensors that don’t agree, lights the engine management light and goes into limp mode. VW, Audi, SEAT and Škoda have a separate EPC light for it. Often the cause is a worn sensor, a corroded plug or damaged wiring rather than the throttle body itself.
• A failing motor or gears. The ECU can tell when the plate isn’t where it asked for, and goes into limp mode. The motor and plastic gears are usually sealed inside, so the fix is normally a new throttle body.
• A sticking cable. On older cars with a cable, a frayed or dry cable can stick, so the revs don’t drop when you lift off. That’s dangerous: knock the gearbox into neutral, brake, and get it fixed.

• Bigger throttle bodies. A larger bore lets more air through when wide open, but on most standard engines the throttle isn’t what limits the air at full throttle, so there’s little or no gain. They make more sense on engines that have been heavily modified to breathe much more air, and need the ECU set up for them.
• Individual throttle bodies, or ITBs, give each cylinder its own throttle, right at the intake port. With so little space between the plate and the valve, the engine responds instantly and makes a loud intake bark. They’re common on race engines and classic tuned cars, and BMW’s M3 of the early 2000s had them as standard. They need careful setting up and a special air filter for each.
• Throttle controllers and “pedal boxes” plug in between the pedal and the ECU and change the pedal map, like the demo’s Sport mode. The car feels sharper, but floored it makes exactly the same power.
• Remaps can change the pedal maps, and on turbo engines they often change how the throttle and wastegate share the job of controlling boost.