Cetane Additive

[MikeT]

While browsing in Wilkinsons I noticed they sell a cetane improver for under £5. So now I'm wondering, how do you judge the efficiency of these things? Even the info on the labelling of all these additives only claim they "can help clean..." or "can help improve..." nothing more concrete than that. I would imagine SVO would burn better with cetane improvement?

Is there a generic product for cetane other than pump fuel?

[steelcityuk]

Interesting thought. I must admit to being wary of these products after seeing the Fifth Gear test of petrol boosters, everyone caused the engine to lose power when tested on a dyno.

I used to use Millers in the 405 TD, back then SVO was cheap enough to ofset the cost.

Steve.

[CitroJim]

Umm, is it actually advertised as a cetane improver Mike? I ask as most diesel additives are heavy detergents and have a beneficial effect of cleaning the fuel system of deposits; thus improving efficiency a bit.

Cetane is a measure of a fuels propensity to spontaneously combust (detonate) under heat and pressure. It is the exact opposite of a petrol octane rating, which is a fuels resistance to combust under the above conditions. Cetane = willingness to detonate whereas Octane = unwillingness to detonate!

Generally, cetane values above 50 do not show any significant improvment in diesel combustion but the real clincher is in another way cetane relates to diesel combustion. That is in the ignition delay value. All diesel fuels exhibit a small delay between being injected and actually starting to burn. The higher the cetane value, the shorter the ignition delay. A fuel with a higher cetane value will allow slightly easier cold starts due to the shorter ignition delay.

This has an effect on how the engine should be timed; the timing being dependant on the cetane value of the fuel the engine is designed to run on during service. Cold fuel has a longer ignition delay than warm fuel and this is the reason a cold diesel engine has its injection timing advanced for cold starting and cold running. As timing is very critical to power and sweet running in a diesel, the cetane value in pump diesel is controlled to ensure that the factory set injecting timing gives optimum power and efficiency.

Now, as to wheter or not a cetane improver is of benefit in veg, you need to know the cetane value of the veg you are using and then add a cetane improver to bring it in line with pump diesel.

I believe most veg has a lower cetane value than most pump diesel and hence why veg users recommend a small advance on pump timing where this is possible on a fully mechanical pump.

[MikeT]

Yes, cetane. 8) First time I've ever seen such an additive. As you say, plenty of cleaning additives, even octane boosters, but never cetane.

I did look for the cetane recommendation in the handbook but it just says diesel - it gives RON numbers for petrol though I guess it's about 50% or more?

[CitroJim]

Generally Mike, pump diesel has a cetane rating of between 40 and 50. Diesels are nowhere as fussy over cetane ratings as petrol engines are over octane ratings, hence no recommendations on the handbook.

Generally winter diesel has a cetane rating nearer the top value to ease starting in the cold.

RON is Research Octane Number and is determined in a test engine running under contitions totally unrelated to typical road engine conditions so the RON value is rather higher than it is in practice. There is another value called MON (Motor Octane Number)which is determined under more "real world" conditions and is much lower. A petrol with a RON or 95 may only have a MON of 82. Some countries (the USA for example) rate their petrol using a PON (Pump Octane Number) value which is an average of the RON and MON and is thus more honest.

Diesel Cetane and petrol Octane values are measured in a test engine where the test fuel is measured against mixtures of pure hydrocarbons whose exact properties are known. Petrol is measured against mixtures of n-heptane, which has an octane rating of Zero and iso-octane which has an octane rating of 100 (the reference fuel). A petrol which exhibits the same behaviour as a 95/5% mix of iso-octane n-heptane mix is rated at 95 octane. The test engine has a variable compression ratio and this is adjusted until dentonation just sets in.

Cetane is measured in much the same way except that the reference fuels are different and the ignition delay value between the reference and test fuels is measured under varying compression ratios.

Neither cetane nor octane can be determined by chemical anlysis of the fuel, hence why test engines are still used.

[DickieG]

I've been using 'Millers' diesel additive for years and found it to be very effective. It claims to raise the Cetane rating as well as lubricate the pump/injectors etc. Whilst I can't confirm the latter, I can confirm that performance is improved, the evidence of this was when I used it in my C15D the engine would always fire on the first turn of the ignition key, without Millers it would turn over several times then fire up so it must be improving the ignition. MPG is also improved to the extent that it pays for itself. I've recommended it to several friends and they've all given positive feedback.