Editing Tachometer (section)

==In automobiles, trucks, tractors and aircraft==

[[Image:c172 g1000 mfd.jpg|thumb|100px|[[Cessna 172]]'s [[G1000]] tachometer (1,060 RPM) and engine hours (1736.7 hours)]]

Tachometers or revolution counters on cars, aircraft, and other vehicles show the rate of rotation of the engine's [[crankshaft]], and typically have markings indicating a safe range of rotation speeds. This can assist the driver in selecting appropriate throttle and gear settings for the driving conditions. Prolonged use at high speeds may cause inadequate [[lubrication]], overheating (exceeding capability of the cooling system), exceeding speed capability of sub-parts of the engine (for example spring retracted valves) thus causing excessive wear or permanent damage or failure of engines. On analogue tachometers, speeds above maximum safe operating speed are typically indicated by an area of the gauge marked in red, giving rise to the expression of "[[redline|redlining]]" an engine — revving the engine up to the maximum safe limit. Most modern cars typically have a [[rev limiter|revolution limiter]] which electronically limits engine speed to prevent damage. [[Diesel engine]]s with traditional mechanical injector systems have an integral [[Governor (device)|governor]] which prevents over-speeding the engine, so the tachometers in vehicles and machinery fitted with such engines sometimes lack a redline.

In vehicles such as tractors and trucks, the tachometer often has other markings, usually a green arc showing the speed range in which the engine produces maximum [[torque]], which is of prime interest to operators of such vehicles. Tractors fitted with a [[power take-off]] (PTO) system have tachometers showing the engine speed needed to rotate the PTO at the standardized speed required by most PTO-driven implements. In many countries, tractors are required to have a speedometer for use on a road. To save fitting a second dial, the vehicle's tachometer is often marked with a second scale in units of speed. This scale is only accurate in a certain gear, but since many tractors only have one gear that is practical for use on-road, this is sufficient. Tractors with multiple 'road gears' often have tachometers with more than one speed scale.  Aircraft tachometers have a green arc showing the engine's designed cruising speed range.

In older vehicles, the tachometer is driven by the RMS voltage waves from the low tension (LT [[contact breaker]]) side of the [[ignition coil]],<ref>{{cite web |url=http://education.yahoo.com/reference/encyclopedia/entry/tachomet |title=Tachometer - Facts from the Encyclopedia - Yahoo! Education |publisher=Education.yahoo.com |access-date=2012-06-05 |url-status=dead |archive-url=https://web.archive.org/web/20121106222358/http://education.yahoo.com/reference/encyclopedia/entry/tachomet |archive-date=2012-11-06 }}</ref> while on others (and nearly all [[diesel engine]]s, which have no ignition system) engine speed is determined by the frequency from the [[alternator (auto)|alternator]] tachometer output.  This is from a special connection called an "AC tap" which is a connection to one of the stator's coil output, before the rectifier. Tachometers driven by a rotating cable from a drive unit fitted to the engine (usually on the [[camshaft]]) exist - usually on simple diesel-engined machinery with basic or no electrical systems. On recent [[engine management system|EMS]] found on modern vehicles, the signal for the tachometer is usually generated from an [[Engine control unit|ECU]] which derives the information from  either the crankshaft or camshaft speed sensor.[[File:Tractor hour meter and tachometer.jpg|thumb|A tachometer on a tractor, reading up to 3000 RPM, with the hour meter below it showing 772.9 hours. The mark at 2500 RPM is the engine speed required to run the [[power take-off]] at 540 RPM.]]

===Traffic engineering===
Tachometers are used to estimate traffic speed and volume (flow). A vehicle is equipped with the sensor and conducts "tach runs" which record the traffic data. These data are a substitute or complement to [[loop detector]] data. To get statistically significant results requires a high number of runs, and bias is introduced by the time of day, day of week, and the season. However, because of the expense, spacing (a lower density of loop detectors diminishes data accuracy), and relatively low reliability of loop detectors (often 30% or more are out of service at any given time), tach runs remain a common practice.