Effect of Cetane on the 6.0L Diesel
 

At work we have been conducting a test on the effect of increasing the cetane of our diesel fuel on our Paratransit Bus fleet. The test fleet are 2005-07 Ford Econoline Chassis with the 6.0L diesel engines. The base fuel is 40-42 cetane and we have been using a commercial additive to increase the cetane by 3 points on a small test group. Of note the powerstroke calls for a 45 cetane for cold weather operation but we missed the cold weather in this test. The buses are not assigned regular routes or regular drivers eliminating those variables from the study, the enhanced fuel group and non enhanced fuel group operate at the same time eliminating weather factors from the test.

Cetane enhanced group
Unit/km/Liters/(L/100km)/MPG (US)
N5010/8123/2104/25.9/9.01
N5058/6524/1428/21.9/10.7
N5062/5072/1542/30.4/7.67
N5067/5490/1261/23.0/10.2
N5089/5812/1242/21.4/10.9
Sum /31021/7577/24.4/9.55

Normal Fuel group
Unit/km/Liters/(L/100km)/MPG (US)
N5055/5490/1505/27.4/8.51
N5090/7880/2046/26.0/8.99
N5072/8571/2213/25.8/9.04
N5073/7624/1871/24.5/9.51
N5060/5603/1271/22.7/10.3
N5061/9052/2125/23.5/9.94
Sum /44220/11031/24.9/9.35

You can draw your own conclusions but we felt unit N5062 may have had other issues and was obtaining abnormally low fuel economy relative to the rest of the test units. If that unit is pulled from the test the cetane group averages 23.3 L/100km or 10.03 MPG. The increase in economy from the cetane was 2.12% counting unit N5062 and 7.26% not counting that unit.

...CETANE is ignition delay under compression ignition.

...so, it stands to reason that more delay means a longer time for the combustion pressure to "work," thus capturing more of the energy (same as with advancing timing for more mpg in SI-engines).

...goal is to capture more "pressure-per-degree" of crankshaft rotation, ie: more torque. And, if you make that happen more often/quickly, you get more HP.

Of note: Advancing the spark timing of an SI engine isn't the actual goal, it's a band-aid for technical design drawbacks. (It wasn't improperly designed, but having been mass produced, its necessary to make a few concessions, in other words). Ultimately, with an SI engine, there should be no pressure build up to and including TDC, and the complete burn should begin, happen, and end through the combustion stroke, with maximum expansion occuring at or near the point where piston acceleration is the fastest, I believe approximately 30* after TDC (probably wrong, can't remember exactly anymore). This is to take advantage of the mechanical advantage which can be gained via the reciprocation/rotation relationship. At some point, the piston begins applying more leveraged force to the crankshaft. At this same point, the amount of energy in the combustion gasses that is being spent is being multiplied by that leverage factor against the crankshaft, thus producing more usable force than is being applied to the face of the piston. For this reason, among others, the maximum expansion of the combustion gasses should occur during the range of peak leverage against the crankshaft to provide maximum usable torque with the least spent fuel.

Advancing the ignition timing makes up for load changes and situational changes as necessary, but does rob some efficiency because for a time, the engine is fighting expansion of spent/burning gasses while trying to further compress the mixture.

The same principles apply to a diesel engine, in part. The gains that were seen by increasing the Cetane rating of the fuel could probably also have been seen by mechanically advancing the fuel racks on the test group to some degree. The added benefit of having done so would be increased power output for a given load/speed without increased fuel use.

No tuning changes were undertaken on the cetane fuel group, these are computer controlled diesels.

10mpg and less..for a six liter diesel.

holy crap.

it could be 3 times that..and proven. that is pothetic. my dads rig is at the 5-7 with 40 tons slapped around...