Features of Combustion Analysis and Diagnostics

The traditional approach to engine development is to measure the air and fuel entering the engine, the gaseous emissions, the torque output, and the noise. By utilizing the features of combustion analysis, the engineer is able to note what is actually happening within the combustion chamber.

Several types of combustion diagnostics are available, some of which are mentioned here and shown in Figures 8.24 and 8.25:

Cylinder pressure measurement

Optical (laser see-through bore and piston single-cylinder special R&D engine) (Figure 8.24)

Optical (luminosity probes) (Figure 8.25)

8 off fiber optic tubes collect the light generated by the

bum for analysis

Figure 8.24 Optical engine for laser experi- ments.

Monitors ionization energy level of bum +ve ...,-

ingflarne propagation:

(a) A VL spark plug, and - -

(b) FE V ion gap plug.

176 An Introduction to Engine Testing and Development

There are a number of experimental so-called "optical engines" that are manufactured by Ricardo Consulting Engineers in Sussex, U.K., which are typical of many. One is a single-cylinder engine with the provision for changing the compression ratio from 4.5: 1 to 25: 1. This engine has a quartz bore and an optical plug in the piston crown.

A laser beam is directed upward through the transparent piston crown via a hole in the crankcase and a mirror that directs the beam upward through the piston. High-speed video cameras using more than one million frames per minute capture the combustion event though the quartz window in the cylinder liner.

The Austrian instrumentation and research company AVL has developed a spark plug (Figure 8.25(a)) that has a series of fiber optic tubes that monitor the flame propagation for further analysis. The German company FEV has developed a ion gap spark plug (Figure 8.25(b)) that monitors the energy level of the airlfuel mixture bum in its early stages.

Utilizing in-cylinder pressure measurement integrated with crankshaft rotation data, the measurement and interpretation of this combustion pressure is used to determine the following:

Piston and crankshaft loads

Torque produced from the burning airlfuel charge equals the IMEP

Torque required to induct the fresh charge and to exhaust the burned charge equals the pumping mean effective pressure (PMEP)

Time required for the combustion flame to develop and propagate Spark timing relative to MBT

Presence and magnitude of knock

Cycle-to-cycle and cylinder-to-cylinder variability

In addition, cylinder pressure combustion analysis can be used to achieve the following:

Assessment of inlet and/or exhaust port and manifold geometries Optimization of the shape of the combustion chamber

Quantifying compression ratio trade-offs, powerlpressure rise/NOx Comparison of spark plug parameters

Selection of valve timing overlap and duration

Optimization of fuel injector timing and on-time (opening duration) Investigation of transient response

Measurement of mechanical friction

Automated mapping (MBT, knock, pre-ignition control) Calibration optimization

Key combustion performance parameters can be summarized as follows:

Mean effective pressure Combustion phasing Cyclic variability Heat release

Combustion Analysis 177

Brake Mean Effective Pressure

The engine output torque at the crankshaft when related to the engine displacement is Work Brake work output (Nm) / Cylinder / Mechanical cycle

BMEP = ^{- -}

Volume Swept volume per cylinder (liter)

Note that the brake mean effective pressure (BMEP) is a measure of work output fiom an engine and not of the pressures in the engine cylinder. The name arises because its unit is that of pressure. Brake mean effective pressure is used to compare the perfor- mance of differing engine capacities and the numbers of cylinders. It is expressed in kilopascals ( H a ) , bar, or pounds per square inch (psi).

1200 x Power (kW) BMEP,,,, =

Engine capacity in liters x Engine revolutions perminute In considering BMEP, it is necessary to refer again to a pressure volume diagram (Fig- ure 8.26) where we can note that work is a function of pressure and cylinder volume.

Indicated Work-lndica ted Mean Effective Pressure

The area enclosed on a pressure volume trace or indicator diagram from an engine is the indicated work done on the gas by the piston (Figure 8.26). The indicated mean effective pressure (IMEP) is a measure of the indicated work output per unit of swept volume, in a form independent of the size and number of cylinders in the engine and engine speed.

Indicated work is made up of the negative work done by the piston due to induction charge compression, plus the positive work done to the piston due to heat release and expansion from combustion, which equals the positive work completed by the cycle, which is the indicated work:

Indicated work output (Nm) per cylinder per mechanical cycle IMEP =

Swept volume per cycle (liter)

Note that the definition of lMEP used here is not universally adopted. Sometimes (most notably in the United States), the IMEP does not always incorporate the pumping work.

This leads to the use of the terms gross IMEP and net IMEP:

Gross IMEP = Net IMEP

+

PMEP where PMEP is the pumping mean effective pressure.

Unfortunately, IMEP does not always mean net IMEP. Thus, it is necessary to check the context to ensure that gross IMEP is not what is intended. The IMEP bears no relationship to the peak pressure in an engine, but it is a characteristic of engine type. The IMEP in naturally aspirated four-stroke spark ignition engines will be smaller than the IMEP of a similar turbocharged engine. This is mainly because the turbocharged engine has greater air density at the start of compression, allowing more air to be burned.

178 An Introduction to Enuine Testinu and Develo~ment

MEP ⁼ W&

Volume

Figure 8.26 Indicated work.

P

CStroke Cycle Process

Work is a function of cylinder volume

BDC

I

TDC V

IMEP has been dealt with at length because it is the keystone to further study. We now shall move on to the other family members.

Pumping Mean Effective Pressure

The following equation can be used to determine the pumping mean effective pressure (PMEP):

Swept pumping work (Nm) per cylinder per mechanical cycle PMEP =

Swept volume per cylinder (liter)

Net Mean Effective Pressure

The following equation can be used to determine the net mean effective pressure (NMEP):

NMEP = IMEP

+

^PMEP

See Figure 8.27 for a summary of IMEP.

The work output of an engine, as measured by a brake or a dynamometer, is more important than the indicated work output:

Brake power = P ~ L A N ' = P ~ ( L A ~ ) N * = P~V,N*

where

L ⁼ piston stroke (m) A ⁼ piston area (m2) n ⁼ number of cylinders V, = engine swept volume (m3)

Combustion Analysis 179

p $

4-Stroke Cycle Process

IMEP

Indicated work output per cylinder per

mechanical cvcle divided

C

TDC

v

^RDC

by the swept\rolume per cylinder

NI = number of mechanical cycles of operation per second (for all pistons)

r

^{- : \}

3

N* =

NZ

⁼ revolutions per second for two-stroke engines, and revolu- tions per second divided by 2 for four-stroke engines

Frictional Mean Effective Pressure and Mechanical Efficiency

To recap, fictional mean effective pressure is

FMEP = IMEP - BMEP Mechanical efficiency is defined as brake power