The movies show a great deal of inventiveness when they depict a character stooping down beside a car and attaching an ignition-triggered explosive device to the vehicle. The resulting explosion and fireball make for good special effects. The bigger the boom, the better.
It would be very difficult to attach an explosive device that uses an external trigger to the underside of a car with a magnet, then walk away and expect it to work. How is the bomb supposed to get its trigger signal? There are no ignition wires on the underside of a car; no wires of any kind usually, except those that are absolutely necessary. This is an area exposed to all kinds of hazards, including water (electrical shorts), salt (corrosion), and stones (impact damage).
To wire an ignition-triggered device requires access to the interior of the car, to tap into the ignition switch or wiring in the steering column, or access to the under-hood area. It is possible to use a triggering device that is not directly connected to the wiring, but any such trigger would be prone to going off accidentally. There are numerous possibilities.
A vibration sensor could be used to detect the engine starting up or running continuously. Someone bumping into the car in a parking lot could trigger the bomb, unless the electronics monitored the duration and frequency of the vibrations to help identify it as the engine.
An acoustic sensor to measure noise would have problems similar to those encountered with vibration, but even worse. Any loud noise could trigger the bomb. If the adjacent car was started and had a noisy exhaust, it could trigger it.
It might be possible to monitor the engine for electrical emissions from the spark plugs when the engine is starting. Again, a small package on the underside of the car could easily be triggered by another source.
While it is possible to wire a bomb into the ignition, access to the car may be difficult to achieve in practice. This explains the popularity of suicide attacks, remote triggers, and timing mechanisms. Suicide attacks are very effective, but always kill the bomber, leaving the organization who initiated the attack constantly on the lookout for new recruits.
Cell phones have become popular, because they can be quickly and easily modified to produce a trigger signal. Dial the phone, it rings, and boom. The bomb is not activated until it is placed on the target vehicle, so the attacker is only exposed for a few seconds to the possibility of a misdialed number.
Radio remote control electronics are also cheap, widely available, and easy to use. But the most common triggering mechanism is a simple clock. A bomb can be easily attached to the car exterior, as long as a steel element is exposed, and set to go off at a specific time. These bombs are usually timed to go off when the target is driving to or from work. Someone with a regular schedule, always leaving home at the same time each day for example, makes a reliable target.
As for the bomb itself, it doesn't have to look like very much. A person can pack a small electronics enclosure with enough semtec to produce a large explosion, along with a triggering device such as a cell phone or electronic clock. Paint the box a medium grey, stick on a few rare earth magnets, and you have a bomb. The underside of a car is a roomy place and a bomb could be attached just about anywhere the attacker can reach, including against the fuel tank or on the inside of the rear bumper, which is made of steel under its plastic exterior. It is very important that the bomb be firmly attached, so it may use more than magnets to secure it.
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