Calibration of Fuels

i) DIESEL ENGINES

Refer to -Air to Fuel Ratios- section to sense the need for a - useful - air fuel ration and hence mixture strength.

In the case of diesel engines, a fuel sprayer is used whereby extremely high fuel pressures (2 to 3 thousand pounds per sq. in. OR 140 to 210 bar) convert the liquid fuel into minute droplets (often described as atomised) which are then projected into the combustion chamber at extremely high speed. The fuel droplets are rapidly raised to ignition temperature and combust, whilst being projected across the combustion chamber, by the very high-temperature compressed air. This is caused during the compression stroke due to the high compressor ratios used by diesel engines (17 up to 25 to 1). The fuel pump has the task of providing the fuel delivery at the fuel-line. Spray rate is decided by the setting of the spring-loaded needle, in the fuel sprayer (called the INJECTOR). The combustion, in such engines, requires only portion of the air, which is available and hence diesel engines can operate with a very wide range of air to fuel ratios. The limiting LEAN mixture is decided by the amount of fuel which causes smoke (i.e. soot as unburned carbon) to be just visible (or tolerated for off-road conditions) in the exhaust gases. Such conditions, of just visible smoke, are defined as SMOKE-POINT CALIBRATION (usually about 22 to 1 by mass).

Calibration of the fuel-injection pump is therefore the adjustment of the maximum flow rate per injection, from the fuel sprayer, and is effected by the rack bar to plunger piston setting on inline pumps and the plunger travel on rotary-distributor pumps.

ii) SPARK-IGNITION ENGINES

a) CARBURATION systems can use fixed submerged jets, supplied by the float chamber, with air bleed into the metered (i.e. calibrated) fuel to attempt to maintain the required air to fuel ratio. Such carburetors are called fixed - choke units. Examples are Solex, Zenith. The alternative is to use a variable-choke arrangement caused by the movement of a piston to the base of which is attached a tapered needle that projects into a single-jet tube which is fed with fuel from the float chamber. The piston is housed in a domed cover such that the depression above the jet tube - which is decided by the air flow rate across the jet tube - is transferred to the upper side of the piston. Atmospheric pressure, under the piston, therefore, causes the piston to rise, lift the needle from the jet tube - and so maintain a sensibly constant depression in the choke tube (i.e. above the jet tube). It is this action which enables a reasonably constant air to fuel ratio to be established for the particular travel of the piston and its associated needle.

Note - With FIXED-CHOKED UNITS no adjustment is provided except for slow-running conditions. Optional jets are purchased with VARIABLE-CHOKE UNITS a limited range of adjustment is provided to the jet tube that affects the entire range of the needle movement. It is normal for three needles to be available, to meet the particular requirements of the engine, i.e. best power, best economy or compromise conditions.

b) ENGINE-MANAGEMENT SYSTEMS
Such arrangements use a constant-pressure fuel pump to supply a fuel rail, with a relief valve (set at approximately 30 p.s.i. or 2 bar) returning surplus fuel to the inlet side of the fuel pump. The fuel can escape from the rail by means of one or more fuel sprayers that deliver the fuel into the induction manifold. Each sprayer has an electrical solenoid operating the sprayer valve; hence deciding the amount of fuel sprayed – into the air stream - by the length of time that the solenoid is activated.

The use of a microprocessor - with influencing sensors measuring many important conditions such as air flow rate at the throttle valve, engine speed, ignition timing, under-the-bonnet air temperatures, coolant temperature, etc - enables a signal (i.e. voltage to be supplied to the sprayers solenoid for a length of time, which in turn dictates the amount of fuel appropriate to the amount of air passing the throttle valve.

Such sophisticated equipment provides distinct improvement in mixture strength, because of the control of the required air to fuel ratio being precise instead of the approximate manner effected by carburetors.