design - Land Rover section


Land Rover FAQs


QUESTION - So what is compression, timing and octane rating?

ANSWER - (By Chris Weinbeck) Compression is exactly what it sounds like, the reduction of the volume of the intake mixture by a certain mechanical ratio. The ratio is dependent on the construction of the engine incorporating the bore, stroke etc. and final volume of the combustion chamber. A 9:1 engine reduces 9 volumes to 1 volume. This compression creates heat -called the heat of compression.

The fuel-air mixture is intended to withstand this increase in heat without self igniting or detonating in a gas engine; instead undergoing controlled ignition by the timed introduction of a spark. A diesel engine depends upon the intake mixture igniting solely due to the heat of compression. The higher the compression ratio the greater the utilization of the energy released by combustion.

Octane rating is intended to quantify a comparison of the particular gasoline to pure octane -the liquid form of straight chain eight carbon saturated hydrocarbons. This liquid was given the rating of 100 (100% octane) early on in the research of internal combustion engines and fuels because of it's excellent ability to resist detonation due to the heat of compression. A given gasoline motor fuel is given a rating (as compared to a mixture octane) based on the average of two "measures" of it's ability to resist pre-ignition. The research method -what all the equations say should be it's "octane rating"- and the motor method -an actual test of detonation due to mechanical compression of the mixture- are averaged to create the octane number posted on the pump labeled R+M/2. Ether,used as starting fluid, self ignites all the way down to 3:1 compression -the practical lower limit in construction of an internal combustion engine.

Timing refers to the adjustment of the points during the compression stroke at which a spark is introduced to ignite the intake mixture. If the fuel-air mixture could ignite instantaneously and fuels with precise octane ratings were available to run in our perfectly constructed engines timing would be permanently set to top dead center "TDC", where the piston is at the exact top of it's stroke and the charge is fully compressed for maximum power and efficiency. But in our less than perfect world it takes an imprecisely known amount of time for the flame front to propagate throughout the mixture of less than perfectly rated fuel and air compressed at something close to a 8:1 ratio. Therefore we are forced to set our timing for something in advance of TDC in most cases, and then use various measures (vacuum, centrifugal force) to determine engine speed and farther increase the "advance" and higher RPM.

Uncontrolled or improperly timed ignition reduces efficiency and can destroy an engine. Detonation/pre ignition can occur at odd locations inside the combustion chamber creating stresses for which the engine wasn't designed. Pre-ignition or improper timing also can cause the force of combustion to peak while the engine is still trying to compress the mixture. Excessively late ignition lowers engine power and causes too much heat to be transferred to the cylinder walls.

Summing it up;

The greater the compression the greater the heat of compression and need for fuels resistant to detonation or pre-ignition.

The greater the speed of the engine the farther in advance of TDC the spark must be introduced to have ignition occur at the correct point in the piston's motion for maximum efficiency.

The higher the octane rating of a fuel the slower it ignites.

The lower the quality (or reliability of the quality rating) of the fuel the farther from peak efficiency timing should be set for controlled ignition. Reply 1999 Christopher Martin, Weinbeck

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