用LabVIEW实现发动机爆震分析仪Building an Engine Knock Analyzer with LabVIEW

用LabVIEW实现发动机爆震分析仪

Knockingin an internal combustion engine is the uncontrolled self-ignition of theair/fuel mixture occurring midway through the combustion cycle, causingextremely high combustion pressure spikes that destroy pistons and rings in theengine. Small amounts of knock (incipient knock) are acceptable in a highlytuned engine, such as might be used in a race car, but the possibility ofincipient knock going into a run-away knock condition due to external stressapplied to the engine must be thoroughly analyzed. The diameter of the cylinderbore determines the primary knock frequency. Secondary knock frequencies arecontrolled by the other dimensions of the combustion chamber, high levelharmonics, and the downward motion of the piston.

Theuniversally accepted system to detect engine knock is an engine combustionanalyzer that measures the gas pressure in the combustion chamber in relationto the crankshaft rotational angle. By using a high pass filter on the pressuresignal or its derivative during the period of combustion, we can accuratelymeasure the intensity of knocking. Each cylinder must have an expensive, hightemperature pressure transducer installed in the combustion chamber, optimizedin location so that the sensor is not in a "dead" area as far asknock is concerned. Since 4 to 10 channels (one for each cylinder) are normallyrequired and a very high speed data acquisition system must be used to performthe analysis in real time, the costs for a complete system typically exceeds$50,000 and the engine must be permanently modified to fit the sensors.

An alternative used by most automobile manufacturers in their productionengines is to use one or more accelerometers mounted on the engine block thatwill sense the high frequency vibrations generated by knock. Unfortunately, thevibrations created in the valve train are typically in the same primaryfrequency range as the knock signal. The placement of the accelerometers iscritical to avoid as much valve train noise as possible and to be as sensitiveto the knock vibrations coming from all of the cylinders. The signal from theaccelerometers is passed through a low and high pass filter. The low passsignal is integrated to make a threshold signal to represent overall vibrationscoming from the engine which are proportional to engine speed. The high passsignal is compared with the threshold signal to determine when knock isoccurring. The vibrations from the valve train cause a great deal of error inthis system at high RPM, due to its inability to distinguish between valvenoise and knock. Additionally, this type system can not detect incipient knock.

Design

Inorder to have an accurate indication of engine knock from a block mountedaccelerometer, the vibrations from the valve train and any other vibrationcausing system (crankshaft and pistons) must be separated from the knocksignal. An IIR filter set from the Signal Processing Library could be usedfor this purpose, but each engine would have different frequencycharacteristics. By using a fast Fourier Transform those frequencycharacteristics may be determined and the appropriate cross over frequenciesmay be applied to the set of IIR filters. This system was fully implemented in LabVIEW and gives excellent results, butrequires a great deal of skill and training on the part of the operator tointerpret the FFT.

Theoperator’s determination of cross over frequencies for each engine could besubstantially simplified by using an averaging fast Fourier Transform. Thecharacteristics could quickly be identified by comparing an averaged FFT at thesame RPM when the engine is audibly knocking to when it is not. The averagingFFT from the NI Sound and Vibration Toolkit was used to make these measurements,averaging over 400 combustion cycles per cylinder. From this information theoperator can accurately determine what unique frequencies to use in the IIRfilter set. Using the graphics capabilities of LabVIEW and the Sound and VibrationToolkit, we quickly and easily developed a display that communicated thenecessary information. The averaging FFT system reduced both the skill level ofthe operator and training time. However, the averaging FFT still depended onhistory to make the cross over frequency determination.

Whatwe ultimately needed was a real time system that was intuitive to the operator.The Sound and Vibration Toolset again came to our aid with one of the mostspectacular displays that is available for FFT analysis. We used the slidingwindow FFT to display the frequency and amplitude relative to time. By using awide range of colors to indicate the intensity of the signal, we make theinterpretation intuitive. By using appropriate examples, we can quickly trainthe operator to identify not only intense knock, but also incipient knock. Thethree dimensional view allows us to easily separate the valve train vibrationsand any other engine vibrations from the knock signal. The best feature of thesystem is the ability to distinguish incipient knock from high intensityknock.. See Figure 4. Note that the combustion cycles with high intensity knockhave tall, bright red, yellow and white "totem poles." The ones withincipient knock have dark blue and purple spots above the main combustion area

Application

Wemodified a 400 horsepower four wheel drive Porsche Twin Turbo to achieve 600+ horsepowerwith all emissions systems operative and running on 93 octane street gas. Itwas capable of a quarter mile acceleration time of mid 10 seconds, top speed of204 mph and weighedin at 3500 pounds. We entered the car, shown in Figure 5, in the One Lap ofAmerica race which included 8 road racing courses and one drag strip, winning 6of these events. Unfortunately, the engine blew up at the event at Pikes Peak,while using 91 octane gas. Of course I blamed the stupid helper who put 91octane gas in it. Little did I realize that it was the stupid engine builder(me) who was to blame.

TheEngine Knock Analyzer revealed the truth! Even with 93 octane gas, the enginehad significant amounts of knock, as we have shown in the screen shots below.We found that the air flow meter was improperly calibrated, causing the engineto knock at high boost levels.

Two combustion cycles of severe knock.

A sliding window FFT showing seven combustion cycles, three withsevere knock.

By using LabVIEW with the Sound and VibrationToolkit, we have been able to develop a real time knock analyzer that with thehelp of the striking visual display makes the determination of knock intuitiveand accurate, and is low in cost.