Car Automotive

Archive for November 19th, 2009

In the development of a new vehicle platform, its crashworthiness is an important concern, and it is imperative to compare the impact severity of the vehicle and occupants under various test and design conditions. Since an impact is a physical event that involves analyses of impulses and energy components, such as kinetic energy, energy absorption, and energy dissipation, the analyses require both the principle of work and energy and that of impulse and momentum.

Although both principles are derived from Newton’s Second Law, they are not mutually exclusive when it comes to solving problems involving impact and excitation.It will be shown that any crash event, modeled by either a single-mass or a multi-mass system,involves impact and/or excitation. Case studies, such as the dynamic principles of pyrotechnic pretensioner on the occupant responses, are investigated. The preloading effect of a restraint system on the occupant response and ridedown efficiency are discussed.

Many crashworthiness topics related to single and multi-vehicle collisions are analyzed by the engineering principles presented so far for determining the degree of crash severity. Applications of these principles to vehicle-to-vehicle compatibility, shear loading of truck body mounts due to eccentric loading, and the methodology of accident reconstruction methodology are also presented.

A crash pulse is the time history of the response of a vehicle system subjected to an impact or excitation. The dynamic characteristics of the system can be described by using a “hardware” or a “software” model. A “hardware” model is a system consisting of masses interconnected by energy
absorbers (springs and dampers).

The present chapter covers the use of a “software” model utilizing digital convolution theory for crash pulse prediction.In a study by Eppinger and Chan [1], the concept of a finite impulse response (FIR) model based on convolution theory is used to assess thoracic injury in a side impact. Using accelerometer data from both the impacting side (rib cage) and the non-impacting side (spine) of a thorax, the torso dynamic system is characterized by a set of FIR coefficients, i.e., a transfer function.

Then, under a different impact condition, the torso response in the non-impacting side can then be predicted by convoluting the FIR coefficients with the accelerometer data for the impacting side of the thorax.The basic operation of convolution theory, the derivation of the transfer function, and an algorithm using a snow-ball effect to increase the computation efficiency are discussed.

Cases are presented which include but are not limited to the (1) Use of transfer functions in assessing the occupant response prediction using various crash pulse approximations, (2) Characterization of truck body mounts by FIR coefficients and the prediction of body pulses with different frame pulses, (3) Evaluation of the performance of air bag and steering column restraint systems for both unbelted and belted occupant responses, and (4) Assessment of sled test pulses and the prediction of its occupant crash severity in a barrier test condition.

Without a good quality spark, in the right place at the right time, the engine performance will be affected, as will the operation of the emissions control system. A misfire can lead to unburnt fuel reaching the exhaust and this will quickly harm the catalyst, often irreparably.

For this reason, modern systems monitor the performance of each cylinder, in relation to combustion. One method of doing this is to ‘sense’ the angular acceleration of the engine flywheel; a firing cylinder will produce more acceleration than a misfiring one. In order to identify the cylinder that is misfiring the ECM requires a reference signal and this is often provided by the camshaft position sensor.

On modern systems, the ECM has the ability to detect misfires because the unburnt fuel that results can cause serious damage to the exhaust catalyst. The ECM achieves this diagnosis by reading the time interval between pulses from the crankshaft speed sensor. Persistent misfires will activate the MIL and a fault code (DTC) will be recorded. Urgent remedial work will then be required if serious catalyst damage is to be avoided.

From time to time, reports circulate about vehicles that have failed an exhaust emissions test. Often, it seems that an assumption has been made that the exhaust catalyst is defective, a new catalyst has been fitted and when retested the vehicle still fails the exhaust emissions test. From this point things can often get worse. It is, therefore, useful to make an examination of the techniques that should prevent such misdiagnosis from happening.

The pre-catalyst oxygen sensor is an important element in the control system that regulates the mixture strength (air–fuel ratio) in spark ignition engines. the oxygen sensor samples the exhaust gas before it enters the catalytic converter and produces a signal that tells the ECM the air–fuel ratio of the mixture that is entering the combustion chambers. It is a feedback system.

The oxygen sensor signal is used by the ECM to change the amount of fuel injected so that lambda is kept in the range of approximately should remind you of the principle.Should the oxygen sensor be disconnected, there will be no feedback to the ECM, which will probably have been programmed to use a substitute value to cause the ‘limp home’ mode to operate and to minimize the possibility of damage to the catalyst. Add to this the fact that, like the catalyst, the oxygen sensor needs to be at an operating temperature of over 300?C for it to operate efficiently, and it becomes evident that electrical testing of the oxygen sensor is work that requires care and attention to detail.