186 An Introduction to Engine Testing and Development
Burn time is critical for a stable idle. A long bum time gives low torque (with low NOx as a trade-off). A short burn time gives high torque.
The potential causes of early burn variability are inadequate ignition (injection) system performance, excessive cyclic airlfuel ratio variability, or excessive dilution (weak charge mixture).
For spark ignition applications, increased spark plug gap gives greater idle cycle-to-cycle stability. For all applications, idle cycle-to-cycle stability is proportional to the degree of valve overlap. High overlap equates with poor idle stability.
Combustion Analysis 187
-
180 0 180 360 540 Figure 8.3 7 CyclicCrank Angle (O) variation.
Figure 8.38 clearly illustrates an example of variation in burn rate. This example is from a standard production engine and was taken over 10 cycles. Note the relation- ship between the rate of burn and the mass fraction burned. The burn rate peaked at a mean figure of 18" crankshaft rotation, and the mass fraction burned completed at 38' crankshaft rotation.
Figure 8.38 Eflect of
-20 0 20 40 -20 0 20 40 60 combustion variability on
Crank Angle ( O ) Crank Angle (") bum rate.
188 An Introduction to Enqine Testinq and Development
How Is Combustion Variability Quantified?
The most common methods to quantify cycle-to-cycle and cylinder-to-cylinder vari- ability include the standard deviation of IMEP and the standard deviation of revolutions per minute.
Standard deviation of IMEP quantifies how widely values are dispersed from the mean, that is,
where i is the sample of interest and n is the number of samples. (This calculation can be performed on an individual cylinder-to-cylinder basis to quantify cyclic vari- ability or on all cylinders to globally characterize engine stability.
The coefficient of variation (COV) of IMEP quantifies variability in indicated work by expressing the standard deviation as a percentage of the mean IMEP as
STDEV of SMEP COV of IMEP =
IMEP
Although opinions vary, degradation in the drivability of a vehicle can be noticed by the driver and passengers when the COV of IMEP exceeds 3-5%.
The lowest normalized value (LNV) of IMEP, an indicator of misfires and partial burn cycles, is determined by normalizing the lowest IMEP value in a data set by the mean as
IMEP min
LNV of IMEP = x 100
IMEP
An LNV of less than 0 indicates a misfire. An LNV of less than 80 indicates a partial burn.
Standard deviation of engine revolutions per minute (revlmin) due to combustion variability.
The IMEP imbalance, a measure of cylinder-to-cylinder variation, is quantified by subtracting the average IMEP in the weakest cylinder from the average IMEP in the strongest cylinder, and then normalizing by the mean IMEP as
There are potential pitfalls when taking the average COV of IMEP, and the data can be misleading (Figure 8.39). From Figure 8.39, where four individual cylinders are considered, the engine average COV of the IMEP is 0% for the IMEP at each cylinder and is constant, whereas the IMEP imbalance is 10.5% (i.e., each cylinder has differ- ing IMEP [kilopascal] levels).
Combustion Analysis 189
Engine average COV of lMEP can be misleading:
n
a 141F.P imbalance = 10.5%
5
0 20
Figure 8.39 Potential pit-
130
40 60 80 loo fall of taking the average
Engine Cycle No.
COV
of the IMEE!The root mean square (RMS) of the AIMEP (i.e., the highest and lowest IMEP read- ings) characterizes the difference in work performed in each cylinder event (in the firing order) as
where nc is the number of cylinders, and x is the number of cycles.
Some Thoughts to Ponder
Do the combustion stability metrics already discussed provide the best measure of combustion stability?
What does the driver feel?
What about the difference in work from each cylinder event in firing order?
Is the phasing of the cylinder events important?
Differential Imbalance Percentage
The differential imbalance percentage (DIP) quantifies the variation in the indicated work done between cylinder firing events by expressing the RMS as a percentage of the mean as
DIP = RMS of AIMEP IMEP
x
100190 An Introduction to Engine Testing and Development
Referring to Figure 8.40, which illustrates the running of an engine more than 3000 cycles at a constant speed, fixed throttle, and controlled coolant and oil temperature, note that there is a wide-ranging IMEP. The mean IMEP is 126.4 kPa, but the standard deviation of IMEP is 15.4 kPa. The COV of IMEP is 12.6%, and the RMS of AIMEP is 24.2 kPa.
When reviewing combustion data, no one set of figures will present the full picture.
RMS of AIMEP = 24.2 kPa
Figure 8.40 Differential 0 500 1000 1500 2000 2500 3000
imbalance. Engine Cylinder Event Nitmber
Filtering the data can be dangerous if not undertaken intelligently. As illustrated in Figure 8.41, filtering of the data from Figure 8.40 shows a stable engine with the IMEP falling away in a stable fashion after 1000 cycles. These data on their own would indicate an engine where one or more pistons were seizing in their respective bores.
That is a totally wrong assumption.
Mean IMEP = 126.4 kPa
Standard deviation of IMEP = 15.4 kPa COV of IMEP = 12.6%
RMS ofAIMEP= 0.122 kPa DIP = 0.096%
I I
Figure 8.41 Filtering the o 0 0 1000 1500 2000 2 . ~ 0 0 3000
data from Figure 8.40. Engine Cylinder Event Number
Some Rules of Thumb
Although these generalities do not always hold true, combustion stability usually improves with the following:
Combustion Analysis 191 Increased speed and load
Higher compression ratios
Lower overlap camshaft timing (valve overlap) Higher energy (at the spark plug gap) ignition systems Higher temperatures
Lower humidity
Unfortunately, there typically is a trade-off between high airflow for power and high in-cylinder motion for increased burn rates and less combustion variability, as shown in Figure 8.42.
6 I Increased Power 1
Figure 8.42 Swirl In-Cylinder Motion trade-ofl
Steps to Improve Stability
The following steps may be taken to improve stability:
Well-balanced combustion system hardware
Equal-length, replicated intake and exhaust runners and ports Replicated combustion chambers (fast burning)
Good exhaust gas recirculation (EGR), air, hel, positive crankcase ventilation (PCV), and purge distribution
Good he1 injectors
- Presenting small droplet size
- With good injector spray targeting (ID1 back of valve, minimize wall-wetting) Healthy ignition system
Highest tolerable energy (radio frequency interference [RFI] is a serious concern) Largest tolerable spark plug gap (spark plug durability)
Lowest-resistance high-tension cables (RFI) Best compromise camshaft
Overlap balanced among high-speed power, low-speed torque, and light load combus- tion stability