Why Hold the Throttle Open During a Compression Test?

Almost every compression testing guide tells you to hold the engine’s throttle wide open during a compression test, but most won’t tell you why it may (or may not) matter.

On gasoline engines, the throttle is a valve, which reduces the amount of air going into the cylinder. Closing the throttle reduces the amount for air sucked into the cylinder during the intake stroke. Having less air in the cylinder means that there is lower pressure at bottom dead center (BDC), so that when the piston moves ‘up’ and compresses the air, the maximum (or peak) pressure is lower than it could have been. This lower pressure means that the readings will be misleading. If you encounter an engine with low compression readings on all the cylinders, you should consider that there may be something obstructing the airflow to all cylinders.

What About Throttle-by-Wire?

Engines with throttle-by-wire (also known as an electronic throttle) may not allow you to open the throttle during cranking. These engines can refuse to start while the throttle is not at the idle position. This is usually because the engine control unit or powertrain control module (ECM or PCM) detects the throttle sensor’s output, and ‘diagnoses’ it as a sensor fault.

Many engines without throttle-by-wire will give you trouble codes if you hold down the accelerator during cranking. This is because the ECM or PCM will detect the throttle’s position, and ‘diagnose’ a sensor fault. In this case, you can either complete the compression test with the accelerator in idle position, or reset the fault code after completing the test.

What About Diesels?

Most diesel engines do not restrict airflow into the cylinders, they control power and speed by using a regulator to adjust the amount of fuel. On these engines, the ‘throttle’ is connected to the engine’s regulator, and will not affect compression readings.

Modern automotive and heavy truck diesel engines use a valve to control engine airflow, and control their exhaust emissions. These throttle valves are open under most operating conditions, and usually won’t affect compression tests.

Does it Really Matter?

Many people do compression tests with closed throttle valves, and get decent results. This is because the throttle body usually does not completely block airflow into the engine. There is usually a small ‘bypass’ passage around the edge of the butterfly valve, or an idle air control valve (IACV), designed to supply enough air for the engine to idle (with no load). Idle speed is usually 5-10 times faster than cranking speed, so the passage or valve usually allows enough airflow to get a compression reading within a few percent of the wide-open-throttle (WOT) value.

The Physics Explanation (without much math)

I mentioned that the throttle’s idle bypass (or IACV) usually allows enough airflow to give you reasonable compression readings, and the physics support it!

If you imagine that the bypass is similar to an orifice (hole), the air velocity is proportional to the square root of the pressure difference from one side to the other. We already know that the cylinder can get suck in a significant amount of air with the throttle ‘closed’ while at idle, enough to maintain speed. Since the engine cranking speed is usually more than five times slower than idle speed, the cylinder can collect much more air during cranking than at idle. Because the amount of air rushing into the cylinder decreases as the cylinder fills (due to the reduction in the (pressure difference), cylinder pressure while cranking will be always less than five times the idle cylinder pressure.

It is impossible to come up with a hard number for how close the closed-throttle cranking pressure will be to the wide-open-throttle (WOT) cranking pressure because of the differences in idle bypasses, cranking speeds, and idle speeds between engines. That said, the closed throttle cranking pressure will be much closer to the WOT cranking pressure than the cylinder pressure while idling; usually a few percent from WOT cranking pressure.

That’s All!

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What are Wasted Sparks?

One of the questions we hear most often is ‘what is a wasted spark?’. Wasted sparks are also known by many other names, including ‘waste sparks’, ‘exhaust sparks’, which adds to the confusion.

Wasted sparks are produced in four-stroke (Otto cycle) engines, where the ignition system produces a spark on every revolution. This is called a ‘wasted spark’ because half of the sparks are created between the end of the exhaust stroke and the beginning of the intake stroke, where there is nothing to ignite, so those sparks are wasted. Wasted-spark systems are simpler than conventional distributor ignition systems, which makes them lighter, and more reliable. Wasted spark systems are most common on small one or two cylinder four-stroke engines, such as those found on lawnmowers, marine outboards, go-karts, and some generators.

Conventional Four-Stroke Engine (Otto) Cycle

  1. Intake (TDC->BDC)
  2. Compression (BDC->TDC)
  3. Expansion (TDC->BDC)
  4. Exhaust (BDC->TDC)

Wasted Spark Four-Stroke Engine (Otto) Cycle

  1. Intake (TDC->BDC)
  2. Compression (BDC->TDC)
  3. Expansion (TDC->BDC)
  4. Exhaust (BDC->TDC)

Why Most Engines Don’t Use Wasted Sparks

Creating and sustaining a spark takes a little bit of energy, usually about 50mJ per spark, but ignition systems are usually only about 1% efficient. Ignition systems waste a lot of energy in powering their various components, so their total energy consumption is about 5J per spark. A small two-cylinder four-stroke engine idling at 1000 RPM will use about 50mL (1.7 oz.) of gasoline per hour just to create ignition sparks. Since waste sparks use almost as much energy as ignition sparks, a waste spark engine will waste almost as much fuel again! And this is just at idle!

Because waste sparks consume energy without producing anything useful, they reduce the engine’s efficiency and power output, while increasing fuel consumption (especially at low power settings). This decrease in fuel efficiency and power output is not acceptable unless the engine’s weight is extremely important.

Do Two-Stroke Engines Have Waste Sparks?

Two-stroke engines do not have wasted sparks, because they do not have an exhaust stroke. On a two-stroke engine, there is a compressed fuel-air mixture to ignite at top-dead-center on every revolution. This simple park timing makes distributors unnecessary on most two-stroke engines. That being said, some large and slow two-stroke engines do have distributors, but these are rare; we have only heard of a few still in use.

Two-Stroke Engine Cycle

  1. Intake/Compression (TDC->BDC)
  2. Expansion/Exhaust (BDC->TDC)

Distributor-Less Ignition Systems (DIS)

‘Distributor-less ignition’ systems (also known as distributorless ignition systems) are those without a conventional high-tension distributor for spark timing. Before the advent of computer-controlled (ECU and ECM) ignition systems, ‘distributor-less four-stroke’ usually meant that the engine had a waste-spark system, with the spark plugs directly connected to a magneto. Modern engines use ECUs and ECMs to control coil-on-plug and coil-near-plug modules, which have one coil pack per cylinder, and no distributor. The bottom line is that distributor-less isn’t what it used to be!

That’s All!

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