Analysis of the In-Cylinder Pressure Oscillogram

The in-cylinder pressure oscillogram is one of the richest sources of diagnostic information.

First of all, one should realize that this oscillogram does not directly reflect any particular parameters of the mechanical part of the engine. It reflects the process of moving gas into the cylinder, which allows us to indirectly assess the operation of the valve train, status of the cylinder-piston group, exhaust system passability, and many other things.

We will now specifically deal with timings of valve opening, closing or overlap. It should be understood that these are not the actual geometric angles determined by the camshaft design. These are characteristic points for the gas-dynamic processes in the cylinder, which give us indirect information only. We should also note that our discussion will touch on pressure oscillograms in the cylinder of an engine running idle at 800-900 rpm.

To obtain pressure oscillograms in the cylinder, the engine should be warmed up to operating temperature, a pressure sensor should be installed on the cylinder in question, replacing the unscrewed spark plug, and the high-voltage wire of this plug should be installed on the arrester. When the engine is equipped with a single ignition module for all cylinders (some Opel, Peugeot and Renault) motors, you can dismantle the module and install additional high-voltage wires between its outputs and plugs, while taking all necessary precautions. If possible, disconnect the connector from an injector of the cylinder being diagnosed, to prevent fuel supply. When taking an oscillogram, it is better to use external synchronization, from the sensor on the first cylinder. Start the engine and make the oscillogram.

Let us consider the segments and characteristic points of oscillograms one by one, mentioning at the same time what information can be derived from their forms and the pressure value.

  • Spark timing
  • Compression stroke TDC (top dead centre)
  • Downward piston motion, pressure drop
  • Timing of exhaust valve opening start
  • Exhaust valve opening segment
  • Exhaust gas release segment
  • Valve overlap
  • Intake segment
  • Intake valve closing timing
  • Exhaust stroke TDC
  • BDC (bottom dead centre)

The maximum pressure in the cylinder corresponds to the top dead centre (TDC). The compression stroke TDC of the cylinder being diagnosed is taken as the zero point of the crankshaft rotation angle.

The first thing to pay attention to is the actual ignition advance angle. The program marks the synchronization time with a thin grey strip, which is nothing but the time of sparking in the cylinder when external synchronization is used.

As an option, you can read a high-voltage oscillogram in the cylinder being examined along with the pressure oscillogram. This demonstrative “picture” of the TDC-to-spark timing relation is an excellent aid when seeking for reasons why an engine fails to start. It should be noted that the angle obtained in this manner is the actual angle and may differ from the angle displayed by the scanner. In case of a large discrepancy, it makes sense to inspect the engine trigger wheel.

The second thing to do before further oscillogram analysis is to make sure that “roughly speaking” there are no serious mechanical problems in the cylinder being checked.

This can be done by comparing pressures at points 1 and 2. An idea of this method is as follows. When gases are compressed by the piston, some portion of them inevitably leaks through cylinder seals, causing the pressure at point 2 to drop relative to point 1. At the same time, the temperature of the gases will increase because of being compressed by the piston and contacting the hot cylinder walls, which increases the pressure. Therefore, the pressure of a fault-free engine at point 1 should be approximately equal to that at point 2. If there is serious mechanical damage in the cylinder (blown valve, broken rings, a fault in the valve train), pressure 1 will be noticeably higher than pressure 2 due to a significant leak of the gases compressed in the cylinder.

This method is most suited for making evaluations, but it is better to make serious conclusions on the condition of cylinder seals using a pneumatic tester.

If the spark timing is correct and there are no obvious mechanical defects detected, we proceed to further oscillogram analysis. We begin with the top dead centre.

The pressure value at the TDC is an integral parameter that depends on many factors. Does this mean that we cannot use it to make a reliable conclusion concerning whether there is any defect? Unfortunately, yes. But still, it is absolutely necessary to understand what this value depends on and to interpret it accordingly.

Here are the major factors affecting the pressure value at the TDC:

1. Engine compression ratio. Naturally, the higher the compression ratio, the higher the pressure. The difference will be noticeable both on engines of different design and on engines of the same make. This is, first of all, due to changes in the compression ratio during operation, e.g. as a result of the accretion of soot in the combustion chamber and bottom of the piston.

2. Absolute pressure in the intake manifold. As the cylinder is filled from the intake manifold through the open intake valve, the amount of gases taken in and thus the pressure at the TDC directly depends on the absolute pressure value. The high value of the latter is mostly a result of air flowing into the throttle-side space. Generally, air inflow can be detected by two signs: high pressure at the TDC and a low vacuum value in the intake manifold.

3. Camshaft status. For example, inlet cam wear will also lead to poor filling of the cylinder and, as a consequence, to low pressure at the TDC.

4. Mixture composition. The optimal mixture composition on which the engine runs most efficiently is the stoichiometric one. Let us remind you that the stoichiometric composition is that in which the air-to-fuel mass ratio is 14.7:1. A deviation from the stoichiometric mixture, either to a richer or leaner mixture, leads to the engine out of its optimal operation mode, decreasing the idling rpms. To maintain the rpms at the required level, the electronic control unit (ECU) opens the idle air control valve (IAC valve) a little. In doing so, the pressure in the intake manifold increases and the pressure at the TDC increases accordingly.

5. Ignition advance angle (IAA). As mentioned above, you should make sure before oscillogram analysis that the IAA is set correctly in order to prevent the IAA from affecting the reliability of our conclusions. Let us explain how the IAA and the pressure at the TDC are related to each other. A deviation of the IAA value from the optimal value, either toward early or late ignition, will decrease the idling rpm value. This also causes further opening of the IAC valve, an increase in the absolute pressure in the intake manifold, and thus an increase in the pressure at the TDC.

6. Status of the cylinder-piston group and valves. The presence of significant gas leaks from the cylinder given an unsatisfactory condition of these assemblies will also result in pressure decreasing at the TDC. But, as mentioned above, approximate assessment of their condition should be carried out immediately once the oscillogram is taken, before performing a detailed analysis.

7. Another critical factor is the number of engine cylinders. We’ll explain using a simple example. The issue is that when an oscillogram is being taken the cylinder in question does not contribute to running the engine. This is one of three cylinders on a three-cylinder motor and one of eight cylinders on an eight-cylinder motor. In the first case, the load on the remaining cylinders increases much more that in the second case. As a result, the IAC valve opens significantly to maintain the idling rpm, which leads to increased pressure at the TDC. Therefore, when studying a three-cylinder Daewoo Matiz, you should not be surprised at a high value for this pressure.

The pressure value at the top dead centre of a fault-free four-cylinder engine varies from 4.5 to 6 bar. Lower values most often indicate that there is serious mechanical damage to the cylinder in question, while larger values show there is reason to look for air inflow or a cause for the high engine load.

The drop in pressure after the TDC corresponds to the downward movement of the piston. The exhaust valve begins opening before the piston achieves the bottom dead centre, which corresponds to a crankshaft rotation angle of 180 degrees. This happens because when the motor is actually run the exhaust gases are under high pressure and, regardless of the increase in cylinder volume, they begin to flow out through the exhaust valve.

In our case, since there is no ignition, when the exhaust valve opens, the in-cylinder pressure is below atmospheric pressure and approximately equal to the intake vacuum. Therefore, when the exhaust valve opens, gases start moving from the exhaust system into the cylinder and the pressure in the latter begins to rise.

The time at which the in-cylinder pressure begins to rise can be conventionally taken as the time when the exhaust valve starts to open. To measure this more precisely, we recommend considerably stretching the Y-axis of the oscillogram.

Then determine the angle from the TDC to the time of exhaust valve opening by using measuring scales. This value lets us to make an unambiguous conclusion on whether installation of an exhaust camshaft on the two-shaft motor or of a camshaft on the single-shaft motor is correct.

On the majority of engines, the exhaust valve opening angle is 140-145 degrees of crankshaft rotation; only some Opel-origin motors have an angle of 160 degrees here. If the angle measured on the oscillogram lies within the specified range, the camshaft is deemed as installed correctly. Remember that we here mean the virtual gas-dynamic angle we observe, while the actual valve opening and closing angles for different motors may vary significantly.

With respect to VAZ motors, resetting the valve train belt by one tooth shifts the valve timing by 17 degrees in the respective direction. In reality, on the oscillogram we will see a shift of approximately 12 degrees for a one-tooth error, 26 degrees – for a two-tooth error; the further the shift, the great the discrepancy observed. This also results from the gas-dynamic nature of the oscillogram in question.

Next. In the segment of the subsequent pressure rise, there is an exhaust valve opening process. This oscillogram segment should be smooth. Irregularities, such as surges or even a “saw” pattern, indicate significant wear of the exhaust valve guide bushing. Exhaust valve vibration when opening causes the pressure to pulse. Below is an oscillogram example of this phenomenon.

At crankshaft rotation of 180 degrees, the piston comes to the bottom dead centre. An oscillogram segment from this point to the 360-degree point corresponds to upward piston movement, to the exhaust system TDC, or TDC 360 degrees. After pressure equalization in the cylinder and in the exhaust system, gas begins to be driving from the cylinder.

At this time, the exhaust valve is open and the piston is moving upwards. In other words, the in-cylinder pressure is actually nothing but the pressure in the exhaust system. This remarkable fact allows us to make a conclusion on the exhaust system passability by properly installing measuring scales and assessing the value obtained.

In this segment a pressure within 0.1 – 0.15 bar is considered to be quite normal. If it is significantly higher, up to 1-1.5 bar, it unambiguously indicates internal destruction of the catalyst or the muffler. Minor increases are also usually the result of some sort of internal damage, though wear of the exhaust valve cam is also possible.

In case of doubt, it makes sense to disconnect the exhaust system joints and take another measurement. This oscillogram segment is especially informative if the idling rpm are elevated, say, to 2000. If there is internal damage to the exhaust system, its pressure will be rather high, up to 2-3 bar.

There are irregularities in the oscillogram segment that corresponds to exhaust gas release. They are caused by wave- and resonance-related processes in the exhaust system. The better the exhaust system has been tuned for the specific engine, the smoother this oscillogram segment will be. Comparing the oscillograms for motors of domestic and foreign manufacturers allows us to make the disappointing conclusion that foreign car manufacturers are much more serious about tuning their products.

Let us consider the top dead centre of the exhaust system, which corresponds to 360 degrees of crankshaft rotation. Shortly before this point, the intake valve begins to open a channel via which the inside cylinder space is connected to the intake manifold. The absolute pressure in the intake manifold is considerably lower than the in-cylinder pressure. As the exhaust valve is still open, the in-cylinder pressure is almost equal to the exhaust manifold pressure. Therefore, for most engines it is not possible to use the in-cylinder pressure oscillogram to detect the time at which the exhaust valve starts to opening.

Speaking of production TDC, we should draw your attention to a characteristic point that corresponds to valve overlap. Here, we mean gas-dynamic overlap when opening areas of the intake valve and the exhaust valve become equal. As diameters of intake and exhaust valve heads are different, overlap takes place at different offset values for these valves.

On some motors, there can be no geometric valve overlap at all. But the virtual gas-dynamic overlap always exists, regardless of engine design. On the oscillogram, this timing corresponds to the beginning of a sharp pressure drop at the end of the exhaust system. For optimal motor operation, the gas-dynamic overlap timing should match the 360-degree mark, which can be observed while examining engines from various manufacturers.

Let’s consider to such a nuance. If it appears during in-cylinder pressure oscillogram analysis that the overlap timing is changing its position from frame to frame, it means the tension of the valve train belt has slackened.

When a piston that has achieved top dead centre changes the direction of its motion, the exhaust valve is almost closed. Therefore, the internal cylinder space disconnects from the exhaust manifold. In doing so, the intake valve continues opening, while the in-cylinder pressure begins equalizing with the pressure in the intake manifold.

As the in-cylinder pressure value is rather high, gases begin to flow from the cylinder to the intake manifold, where the pressure is much lower than atmospheric pressure. Soon, pressures in the cylinder and in the intake manifold become almost equal. Meanwhile, the piston is moving downward, the intake valve is open, and the pressure value in the intake segment is nothing but the vacuum in the intake manifold. Its averaged value in a fault-free motor is 0.6 bar.

A lower vacuum value is reason to look for the source of the defect. Unfortunately, the intake manifold vacuum, like the compression TDC pressure considered above, depends on a variety of factors. Small damped oscillations in the intake segment occur presumably due to resonance processes in the intake system.

Having achieved the bottom dead centre of 540 degrees, the piston starts moving to the cylinder head again. But the intake valve is still remains open for a while. Let us explain why. The process of moving gas from the intake manifold to the cylinder has a significantly slow response and, despite the fact that the piston is moving toward the TDC and the cylinder volume is decreasing, the cylinder continues to fill through the open intake valve due to the stream’s inertia. The delay in closing the intake valve is intended to improve the rate at which the cylinder is filled with the fuel-air mixture.

This effect depends on the crankshaft speed and throttle opening rate. The intake valve closing time should be selected in such a way that cylinder “milking” would be maximum at a specific rpm value and with the fully open throttle. If the engine runs at a low crankshaft speed, the effect from late intake valve closing is negative: a portion of gases will flow back to the intake manifold.

The intake valve closing time on the oscillogram can be seen only roughly:

  • At idle speed (800-900 rpm), when gases are flowing from the cylinder to the manifold at the valve closing time, this will be the time at which the pressure will start to rise.
  • At high rpms, when the cylinder “milking” process happens at valve closing time, a small gap in the graph will be seen. This gap appears because the pressure before full valve closing increased due to compression and “milking”, while after the valve was closed – only due to compression. Ideally, there shouldn’t be any bump at all, but to achieving this is not possible for actual commercial motors.

The intake valve closing time on the pressure oscillogram should be at approximately 580 degrees. You can establish whether installation of the intake camshaft on the two-shaft engine is correct based on the valve overlap position and the intake valve closing time.

Once the intake valve is fully closed, the piston is moving to the compression stroke TDC, and the cycle repeats itself from the beginning.

LET’S SUMMARIZE

The in-cylinder pressure oscillogram allows us to determine:

  1. The actual ignition advance angle based on the relationship between TDC and the high-voltage pulse.
  2. The condition of the mechanical portion based on pressure difference before and after compression (approximately).
  3. Whether installation of the exhaust camshaft is correct based on the exhaust valve opening angle.
  4. Whether installation of the exhaust camshaft is correct based on the valve overlap position and the intake valve closing timing.
  5. The condition of the exhaust valve guide bushing based on the shape of the oscillogram.
  6. The exhaust system passability based on the pressure value at the time of gas release.
  7. Vacuum presence and value in the intake manifold.
  8. Presence of a slack valve train belt based on the frame-to-frame difference of valve overlap angles.