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Discussion Starter · #3 ·
the mechanical timing isn't changed, but the fuel is injected and burns differently than diesel. from another forum

"You are correct about the ignition delay. As long as one is not running Propane, ignition does not occur until the injector fires in a diesel. But higher cetane does result in peak CPs being reached sooner as the fuel burns faster. So the CP peak occurs sooner effectively adv. timing.

I've done CP testing and tuning on my 7.3 and higher CPs do not automatically create more hp. Somtimes the CP peak occurs so soon that the peak that should occur at around 4 degrees ATDC for maximum hp/ torque/ mpg actually occurs when the pistin is still coming up. This results in very high CPs and LESS hp. It also breaks parts.

I've watched a PSD truck get driven cross country for CP tuning that got 13 mpg on the way to the shop. CPs were lowered from 3200PSI occuring at 1 BTDC to 2400psi occuring at 5 ATDC. Torque went up around 90 pounds, hp up 20, and the truck got 18 mpg on the way back home. The injection timing that results in maximum torque is called MBT (Maximum Braking Torque) and always results in maximum mpg. Also, for the given power produced the least strain on the motor occurs when injection timing maximizes MBT.

MBT is not when the injection event should begin but it is where the main CP spike should occur. It is where the piston is still close enough to TDC yet the burn is really peaking out.

The text book MBT is 4 ATDC but high revving motors like less and slow revving motors like more."

In other words, the fuel is burning more readily and more smoothly when it is injected. Here is a more detailed explaination from chevron

[FONT=Arial, san serif]CETANE NUMBER [/FONT]
[FONT=Arial, san serif]Cetane Number is a measure of how readily the fuel starts to burn (autoignites) under diesel engine conditions. A fuel with a high cetane number starts to burn shortly after it is injected into the cylinder; it has a short ignition delay period. Conversely, a fuel with a low cetane number resists autoignition and has a longer ignition delay period. ) Although the cetane number of a fuel is assumed to predict its ignition delay in any engine, the actual delay represented by the cetane number is valid only for the single cylinder engine in which it was measured. The fuel's performance in other engines may differ. [/FONT]
[FONT=Arial, san serif]
A fuel's ignition delay is determined by its chemistry. In a warm engine, the delay is independent of the physical characteristics of the fuel, like volatility and viscosity. [/FONT]

[FONT=Arial, san serif]POWER[/FONT]
Power is determined by engine design. Diesel engines are rated at the brake horsepower developed at the smoke limit.For a given engine, varying fuel properties within thes pecification range does not alter power significantly. For example, in one study seven fuels with varying distillation profiles and aromatics contents were tested in three engines. In each engine, power at peak torque and at rated speed (at full load) for the seven fuels was relatively constant. However, if fuel viscosity is outside of the D 975 specification range, combustion may be poor, resulting in loss of power and fuel economy.

[FONT=Arial, san serif]NOISE[/FONT]
The noise produced by a diesel engine is a combination of combustion noise and mechanical noise. Fuel properties can affect only combustion noise.
In a diesel engine, the fuel ignites spontaneously shortly after injection begins. During this delay, the fuel is vaporizing and mixing with the air in the combustion chamber. Combustion causes a rapid heat release and a rapid rise of combustion chamber pressure. The rapid pressure rise is responsible for the diesel knock that is very audible for some diesel engines.
Increasing the cetane number of the fuel can decrease the amount of knock by shortening the ignition delay. Less fuel has been injected by the time combustion begins and it has had less time to mix with air. As a result, the rapid pressure rise, along with the resulting sound wave, is smaller. One design approach to reducing combustion noise is to shape the injection – setting the rate slow at first and then faster – to reduce the amount of fuel entering the cylinder during the ignition delay period. Another is to use indirect-injection

[FONT=Arial, san serif]FUEL ECONOMY[/FONT]
Here again, engine design is more important than fuel properties. However, for a given engine used for a particular duty, fuel economy is related to the heating value of the fuel. Since diesel fuel is sold by volume, fuel economy is customarily expressed as output per unit volume e.g., miles per gallon. Therefore, the relevant units for heating value are heat per gallon (Btu per gallon). Heating value per gallon is directly proportional to density when other fuel properties are unchanged. ASTM specifications limit how much the heating value of a particular fuel can be increased. Increasing density involves changing the fuel's chemistry – by changing aromatics content – or changing its distillation profile by raising the initial boiling point, the end point, or both. Increasing aromatics is limited by the cetane number requirement (aromatics have lower cetane numbers ; changing the distillation profile is limited by the 90% distillation temperature requirement. Combustion catalysts may be the most vigorously promoted diesel fuel aftermarket additive. However, the Southwest Research Institute, under the auspices of the U.S. Transportation Research Board, ran back-to-back tests of fuels with and without a variety of combustion catalysts. These tests showed that a catalyst usually made "almost no change in either fuel economy or exhaust soot levels."2 While some combustion catalysts can reduce emissions, it is not surprising that they don't have a measurable impact on fuel economy. To be effective in improving fuel economy, a catalyst must cause the engine to burn fuel more completely. But there is not much room for improvement. With unadditized3 fuel, diesel engine combustion efficiency is typically greater than 98%. Ongoing design improvements to reduce emissions are likely to make diesel engines even more efficient.

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