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Understanding Running Compression

By Lee Hearnden - LMH Engineering Services

This Technical Article is based upon the Running Compression waveform that is produced when using the PicoScope Automotive Oscilloscope and the WPS500X Pressure Transducer.

This allows us to see and determine the overall mechanical condition of the engine and if there are any issues inside the cylinder when the engine is operating.

In order to complete a Running Compression Waveform you will need the following equipment:-

  • PicoScope Automotive Oscilloscope.
  • WPS500X Pressure Transducer
  • BNC to BNC test lead (Supplied with the WPS500X)
  • Compression Hose

Setting up the PicoScope Hardware

Channel A - Running Compression

Connect the WPS500X Pressure Transducer to Channel A of the PicoScope as shown in the Illustration below using the BNC to BNC test lead.

Ensure that the WPS500X has performed the self-calibration process and is set to 'Range 1' for the test.

Remove the 'HT lead' or 'Ignition component' and then remove the Spark Plug from the cylinder which you wish to test. Then using the correct WPS Compression Hose, Connect the WPS500X to the engine accordingly.

Connection Diagram

At LMH Engineering Services, we performed this test on a 1999 Ford Mondeo.

The following image shows the WPS500X Pressure Transducer connected to

Cylinder 1 on the 1.8 Zetec Engine.

Connection Image - Ford Mondeo 1.8 Zetec Engine

You can see in the image above that due to the long threaded spark plug body construction we have removed the collar from the M14 Compression Hose so that the whole threaded body of the Compression Hose can take up as much of the 'Spark Plug' cavity in the Cylinder Head. This will ensure that the readings achieved are as accurate as possible by ensuring the Compression Volume isn't increased.

NOTE: Selecting the correct Compression Hose depth [Applies to M14 Compression Hose Only]

Always make sure that the threaded body of the Compression hose is never longer than then threaded body of the Spark Plug. Most modern engines are 'Interference Engines' i.e. the Pistons Crown at TDC is extremely close to the Valves and Spark Plug Electrode. If you were to insert the Compression Hose to that of a depth more than the Spark Plug it could hit the Piston Crown and cause severe damage to both the Compression Hose and the components in the engine.

Isolate Ignition

You can also see from the image above we used a grounded spark tester to prevent any arcing across the ignition coil therefore preventing any damage occurring and safely allowing the High Voltage Spark to go to ground.

Prevent Fuel Supply

You must also ensure that the Fuel is cut off to the cylinder you are going to test. This is important as there is no spark hence no combustion so un-burnt fuel will be able to pass through the cylinder and into the exhaust system.

This can cause premature failure of the Catalytic Converter.

You can see we simply disconnected cylinder 1 Fuel Injector.

Example Running Compression Waveform

Click here to download Running Compression Waveform

Before looking at analysing the Compression Waveform it's a good idea to re-familiarise the 4 Stroke Cycle and take a look at the engine used for the test to assist with this Technical Article.

Engine 4 Stroke Cycle Familiarisation

The 4 Stroke Cycle also known as the OTTO Cycle occurs over 720 degrees of Crankshaft Rotation.

The 4 Stages are as follows:-


The Engine induces a fresh mixture of air/fuel into the cylinder through the open Intake Valve as the piston travels from TDC to BDC.


Both the Intake and Exhaust Valve are closed and the piston is travelling from BDC to TDC compressing the mixture ready for combustion.


The Ignition source ignites the mixture starting the combustion process. Power is then transferred to the piston increasing the inertia as it travels from TDC to BDC.


The Exhaust Valve is open and the piston is travelling from BDC to TDC pushing out the burnt mixture.

Piston Velocity

When discussing running compression there are factors that need to be taken into consideration alongside the 4 stroke cycle process.

One of these is piston velocity, or piston speed. It's important to understand that when the piston is at TDC the piston is at standstill with a velocity of 0 and also at BDC the piston is at standstill, with again 0 velocity. The point at which the piston is at full velocity is approximately at each 90 degree interval in-between, as shown by the graph below.

The mean piston speed is restricted due to the resistance of the gas flow within the cylinder and also the mechanical stresses caused by the inertia of the moving parts. The mean piston speed generally speaking operates within a maximum range of approximately 8 - 15 m/s, where automotive engines operate nearer the 15 m/s range and large commercial diesel engines operate nearer the 8 m/s range. The effect of piston velocity will be seen and explained later alongside running compression.

Graph calculated on the following: Crank Angle = R = 3.5

Where R = l/a

       l = Length of Con-rod

       a = Crank Radius

Valve Timing Information

On the 1.8L Zetec Engine we decided to measure the Camshaft Profiles which will give us a pretty good indication when the valves are opening and closing and the approximate amount of Valve Lift. It's important to remember that as accurate as we measured these, they don't give us the exact results but are a good indication of events.

The reason for this is because, like most modern engines, this engine is equipped with hydraulic valve followers, or hydraulic tappets as more commonly referenced too. Although the tappet is designed to operate the valve relative to the position of the camshaft profile, it's important to understand that there will be a slight delay, or lag and reduction in lift. This is one of the key reasons these types of engines have no measurable valve clearances.

The reason our results are approximate is when the engine is running the tappet partly relies on increased oil pressure from the trapped oil which is supplied from the oil pressure system. It also relies on the inertia force from the downward thrust caused by the camshaft flank period to close the oil pressure chamber via the ball-valve. Therefore due to moving components i.e. plunger configuration, and the waiting time for the ball-valve to shut down onto its seat, causing the oil to become trapped, there will be a delay time for the tappet to close and allow the increased oil pressure to form the hydraulic tappet to then operate the valve accordingly.

Remembering that the running conditions of the engine are constantly fluctuating and whilst the engine speed is maintained at a steady idle speed the mechanical operating conditions aren't constant so we can only use our results as a reference.

Inlet Camshaft Profile being measured.

Exhaust Camshaft Profile being measured.

Now as we know that the camshaft rotates half the speed of the crankshaft, we decided to take measurements at every 1 degree of camshaft rotation i.e. every 2 degrees of crankshaft rotation.

From here we were able to plot the camshaft profiles, which would give us the valve lift and the camshaft opening duration. Here is a graph showing the results for the 1.8L Zetec Engine.

Where: -                 Exhaust Camshaft Profile                  Inlet Camshaft Profile

From this we were able to determine the following information:-

Note all measurements are respective to crankshaft rotation.

Inlet Camshaft Profile

  • Opening: 29 degrees BTDC (Intake Stroke)
  • Closing: 59 degrees ABDC (Compression Stroke)
  • Maximum Valve Open Period: 268 Degrees
  • Maximum Valve Lift: 8.9 mm
  • Maximum Valve Lift Hold Duration: 8 Degrees

Exhaust Camshaft Profile

  • Opening: 49 degrees BBDC (Exhaust Stroke)
  • Closing: 19 degrees ATDC (Intake Stroke)
  • Maximum Valve Open Period: 248 Degrees
  • Maximum Valve Lift: 7.8 mm
  • Maximum Valve Lift Hold Duration: 8 Degrees

Valve Overlap

  • 48 Degrees

Using the running compression waveform we took for this engine we then aligned the valve timing information giving us the best accurate analysis possible.

Running Compression Analysis

To be able to analyse the waveform we have added some useful information and timing of events to help you visualise and understand what is being seen from the waveform itself.

This waveform was taken whilst the engine was at idle speed. We have made an overlay of our Cam Profiles with the Running Compression Waveform to make it clearer.

It can be seen by looking at the image above that we have identified the 'Maximum Pressure Peaks' showing the pressure that has been created within the cylinder. It's important to note that as there is no fuel and spark present during this test, the cylinder is effectively acting like an air pump so only pressure from the compression of induced air can be seen. We have also overlaid the Valve Timing results we obtained from the Ford Mondeo.

  • At TDC (0 degrees rotation - Power Stroke)

At this point the piston is at TDC after completing the compression cycle but as there has been no combustion the compressed air is then forced to expand and reduce in pressure as the piston travels back down to BDC. This can be seen by looking above as the pressure peaks at 6.7 bar and then just before the exhaust valve opens the pressure is reduced to -730 mbar.

The total amount of pressure created within the cylinder will be dependent upon the following factors:-

  • The overall condition of the engine.
  • The condition of the compression rings.
  • The condition of the valves and valve seats.
  • The Clearance Volume of the engine.

At TDC the peak pressure point represents the position of the piston within the cylinder and that it has reached the end of the stroke. At this point the piston is at a complete standstill and is ready to return to BDC upon the next stroke. This also indicates that the Crankshaft has also reached the end of its stroke governed by the big end and con-rod assembly and at this point, and due to no combustion, the pressure will not increase past the maximum permitted unless the running of the engine changes i.e. increase in engine speed, throttle position.

  • Mid Compression Point

The mid Compression point is the position where the pressure is half of the total maximum pressure achieved. This should be at approximately 30 degrees ATDC and is usually directly half way down the compression part of the waveform.

  • Piston Travelling from TDC to BDC (0 - 180 degrees)

As the piston is moving away from the mid compression point, towards BDC. The volume of the cylinder is constantly increasing as the piston moves further away from the cylinder head, formed combustion chamber, until it reaches its next stopping point at BDC. As the piston is travelling down the cylinder the velocity, or speed of travel, will also continue to increase as the crankshaft rotates from 0 degrees, TDC, to 90 degrees. From 90 degrees to 180 degrees, BDC, the piston's speed will decrease as the piston comes to the end of its next stroke.

Ford Mondeo Exhaust Valve Opens at approximately 49 Degrees BBDC as measured.

It can be seen that within the first 90 degrees of crankshaft rotation, the cylinder pressure has gone from 6.7 bar to (-) bar indicating all pressure within the cylinder has fully de-pressurised As the Exhaust valve at this point is still closed and there has been no combustion, the pressure within the cylinder is in a stage of depression, or vacuum as its more commonly referred to.

The piston continues to travel down the cylinder towards BDC; towards 180 degrees crankshaft rotation. The amount of vacuum continues to increase as the volume is still increasing within the cylinder and the air contained inside the cylinder is forced to expand and 'stretch' to fill the volume, thus increasing negative pressure.

During this part of the cycle, the Exhaust Valve begins to open within the valve timing of the engine. This can be seen by looking at the waveform and even though the piston is still travelling towards BDC the pressure now begins to rise out of a vacuum towards 0 bar, causing an equalisation of the cylinder.

The key reason for this increase of pressure is due to the pressure in the exhaust system being higher than a vacuum, caused by back pressure within the exhaust. The pressure will continue to rise out of a vacuum until it reaches the pressure of the exhaust causing full equalisation. The change in pressure should start to build up a lot quicker when the piston has reached BDC, hence like TDC the piston has reached a standstill and is at zero velocity.

The area of the waveform where the pressure rises can be referred to as the exhaust equalisation ramp,see waveform below.

Equalisation Ramp caused when the Exhaust Valve opens.

NOTE: The equalisation ramp period should start and finish with the approximate mid-point being at around 180 degrees, BDC. With this in mind this indicates that the exhaust cam timing is correct for the set-up of the crankshaft configuration. As long as the period of equalisation falls roughly between

-10 degrees to + 15 degrees of the BDC position then this indicates that the camshaft for the exhaust valves is correctly timed to the piston position.

This should fall in line with the timing marks on the engine or if there are none present then this is a good indication that the exhaust timing is correct for the engine being tested.

High Performance Engines

It's important to note and understand that on some high performance engines the timing of the exhaust valves can be advanced for the running requirements of the engine. The timing though can still be within the time of +20 degrees of the target, BDC.

Image sourced from Wikipedia

This article is based upon the Naturally Aspirated engine fitted to the Ford Mondeo.

  • Piston Travelling from BDC to TDC (180 - 360 degrees)

The piston at BDC is positioned at the bottom of the cylinder and is at its standstill position. As the crankshaft continues to rotate the piston is quickly accelerated from 0 to full velocity and begins to travel up the cylinder, now on the exhaust stroke. On this stroke the exhaust valve is in the open position, as the piston travels up the cylinder it forces and pushes the air out of the cylinder and into the exhaust system.

This can be seen by the pressure pulses showing along the exhaust stage indicating that the air is being pushed out past the Exhaust Valve and the turbulence generated as it rushes into the exhaust system,see waveform below

Pressure pulses created upon the Exhaust stage.

As the piston reaches the mid travel on the stroke i.e. 270 degrees crankshaft rotation, it's now at full velocity again and once past this position it starts to slow again due to the crankshaft throw. As the crankshaft approaches a period of 330 - 345 degrees, 30 - 15 degrees BTDC, the Intake valve will begin to open. As the Inlet valve begins to open, the pressure in the Induction manifold causes a reaction and a pressure change within the cylinder. This can be seen looking at the waveform below.

Ford Mondeo Intake Valve Opens at approximately 29 degrees BTDC as measured.

Pressure change created when the Inlet Valve opens.

Once the piston reaches TDC, 360 degrees, it's now at rest point again, 0 velocity.

  • Piston Travelling from TDC to BDC (360 - 540 degrees)

At this point the Inlet Valve is opening and the piston is at 0 velocity. As the Exhaust Valve is still open, valve overlap period, the cylinder is equalised to that of the higher pressure, this is the pressure within the exhaust system.

Ford Mondeo Exhaust Valve Closes at approximately 19 Degrees ATDC as measured.

As the piston begins to move from TDC towards BDC i.e. down the cylinder, the force from the piston generates enough negative pressure, i.e. suction, to overcome the pressure in the exhaust system and the cylinder begins to become de-pressurised The pressure will continue to drop with the on-going piston downward travel until it reaches and equalises with the pressure in the inlet manifold. As the Inlet manifold is on the 'suction' side of the engine i.e. pre-combustion, the manifold is always in a vacuum pressure unless boosted by a Turbocharger or Supercharger.

The result in this de-pressurisation can be seen and identified on the waveform as an Inlet Equalisation ramp, this can be seen on the waveform below.

Equalisation Ramp caused when the Inlet Valve opens.

Shortly after the Exhaust Valve closes leaving the Intake Valve open whilst the piston is still travelling down the cylinder. Within approximately 60 degrees after TDC the pressure within the cylinder should have fully equalised with the pressure of the Induction system. By taking the mid-point of the Inlet Equalisation ramp or half of the pressure between the start and end, this point should take place at approximately 20 degrees after TDC, see waveform below.

Intake Equalisation timing within Running Compression

This is a good indication that the Inlet Camshaft or Inlet Valve Timing is timed correctly to the crankshaft configuration. If the Intake equalisation ramp falls within the 20 degrees after TDC approximately plus or minus 10 degrees of this region, then this indicates that the Intake Valve is

timed correctly to the piston of the engine.

Overall the Inlet Valves are giving the best opening duration possible.

It can be seen by looking at the waveform that there are pressure fluctuations whilst the Inlet Valve is open; this is the air being subjected to turbulence as its being induced in past the Inlet Valve. As the WPS500X is a sensitive Pressure Transducer it allows you to see all the detail of events clearly. It can also be seen that as the piston velocity reduces towards BDC, the fluctuations decrease as the induction, vacuum effect begins to also reduce.

  • Piston Travelling from BDC to TDC (540 - 720 degrees)

With the Intake Valve still open at this point the piston is at BDC and about to begin its upwards travel to TDC (720 degrees). By looking at the waveform it can be seen that the pressure increases relative to piston position but at a gradual rise as the Inlet Valve is still open. The Intake valve should close at approximately 50 - 60 degrees after BDC.

Ford Mondeo Intake Valve Closes at approximately 59 degrees ABDC as measured.

Once the Inlet valve has closed and the cylinder has become fully mechanically sealed, the piston continues to travel up the cylinder gaining velocity through the inertia generated from the crankshaft until at 630 degrees when full velocity has been reached. The air begins to compress more rapidly as shown through the compression ramp-up period. Once 630 degrees has passed the pistons velocity begins to decrease as the stroke has passed the half-way point. At approximately 690 degrees, the compression should be at the mid-compression point indicating that half the achievable pressure has been generated within the cylinder. The pressure continues to build up even though the piston is slowing due to the Compression ratio and won't stop building up until the piston has come to a standstill at 720 degrees, TDC.

It's important to understand, and it can be seen by looking at the waveform that most of the pressure generated in the compression phase is generated within the last 30 degrees of piston travel.

Once at TDC, 720 degrees, the cycle repeats itself through all the events as mentioned above.

Please note this article was constructed around the Naturally Aspirated Engine and does not cover advanced Technology such as VVT and Forced Induction.

This Technical Article is included in our WPS500 Training booklet which is

supplied FREE with the purchase of the WPS500 Full Kit.

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