Cylinder pressure measurement

What is a cylinder pressure measurement?

Cylinder pressure measurement is the basis for cylinder pressure indication: a metrological method to measure and analyze the pressure curve inside the cylinders of reciprocating piston combustion engines.

Because of the high pressures involved, measurement of the internal cylinder pressure is also known as 'high-pressure indication'. 'Low-pressure indication' is an additional type of cylinder pressure measurement. It is carried out during the gas exchange phase in order to determine the pressure in the intake and exhaust systems. So that the measured pressure can be assigned to a specific working phase of the combustion engine in each case, the piston position (crank angle) or time are included in the calculation.

These methods deliver data that is required for engine research, development, and tuning. They also provide the essential basis for engine manufacturers to comply with increasingly strict exhaust gas legislation and to optimize the efficiency of their engines. 

The cylinder pressure curve determined by means of cylinder pressure measurement is the most important source of information for cylinder pressure indication. Cylinder pressure indication yields more accurate knowledge about the thermodynamic processes during combustion and the engine power that is delivered. The effects of actions to optimize the engine based on this knowledge are:

• Improved efficiency
• Higher engine power/performance
• Reduced emissions
• Longer engine lifetimes

Where is cylinder pressure measurement used?

Cylinder pressure measurement is used for:

• Automobile, motorcycle and commercial vehicle engines
• Large marine engines, e.g. 2-stroke and 4-stroke diesel engines in the shipping industry
• Stationary large engines such as high-power engines for power plants

Which measurement technology is used for cylinder pressure measurement?

Cylinder pressure measurement is mostly performed with piezoelectric high-temperature pressure sensors that are installed through a mounting bore which has to be drilled in the cylinder head for this specific purpose. Measuring spark plugs with an integrated high-temperature pressure sensor are also used. They do not require a mounting bore because they can easily be screwed in instead of a standard spark plug. On diesel engines, special glow plug adapters can also be used for the measurement.

The measuring chain is completed with a charge amplifier, a data acquisition system and an evaluation system. In the automotive sector, there are also innovative indication systems that combine data acquisition and evaluation in one device; these can be used on test stands and also as mobile applications. 

Why is it so important to measure the cylinder pressure curve?

The cylinder pressure curve determined by means of cylinder pressure measurement is the most important source of information for cylinder pressure indication. In principle, reciprocating piston combustion engines are heat engines: by means of combustion, they essentially convert the chemical energy bound in the fuel/air mixture into mechanical work and heat. 

Developers aim to obtain the highest possible proportion of mechanical work from the conversion process – in other words, their goal is to maximize efficiency. Significant factors here are the level and curve of the cylinder pressure over the crank angle, acting on the piston. This pressure curve represents the combustion, so it indicates how energy is being converted in the engine. The total mechanical work on the piston summed during one combustion cycle or stroke is obtained from the pressure and the related change in volume of the combustion chamber.

What are the characteristic variables for the cylinder pressure curve?

The key characteristic variables are the signal level (peak pressure) and the indicated mean effective pressure (IMEP) over one combustion cycle.

How is optical combustion analysis used to measure cylinder pressure?

Optical combustion analysis is used as a complement to cylinder pressure measurement and other options for optimizing combustion processes. The basis for this is provided by state-of-the-art optical probes that precisely detect the origins of combustion knock and pre-ignition events, as well as the formation of soot in the combustion chamber. These optical accesses can be integrated into spark plugs of all types. Other systems can integrate high-speed camera images to visualize fast subsystem processes such as injection events and flame propagation.