The harsh winter temperatures that have hit the country
The harsh winter temperatures that have hit the countrythis winter – minus 5˚C in Vereeniging as this article is written –have caught truck operators struggling with cold start problems. FleetWatch technical correspondent, Dave Scott, looks at the issue of cold starting and diesel freeze in an effort to ensure operators do not get caught frozen.
Does diesel fuel freeze – at what temperature? Are downtime reports from the field accurate when it’s perceived to be a fuel problem – or is it a contaminant such as water that is freezing? Are we assigning coldstart problems to fuel when it’s really a low-amperage electrical problem? Are we adding in uncertified specification bio-fuels that changes diesel’s cold filter plugging point (CFPP)? What is ‘cloud point’ and CFPP? It’s more complex than you think.
At the outset let’s remove the myth of the ‘frozen battery’. Says failure analysis expert, Patrick Swan: “A fully charged battery freezes at minus 68˚C, while a partiallydischarged battery will freeze anywhere between minus 17˚C and minus 27˚C and a fully-discharged battery freezes from minus 3˚C. It’s probably electrical resistance, especially corroded terminals that are dropping amperage to crank a cold engine.”
Diesel fuel’s cold-flow performance is a key measurement and requirement. So why regulate this specification? According to a SAPIA (South African Petroleum Industry Association) publication, Petrol and Diesel in South Africa, ‘cold flow properties directly influence cold temperature vehicle operability… specifications provide a measure of consumer protection to ensure the fuel will provide trouble-free operation in the specific climate conditions expected in the region. As cold flow properties are related to climatic conditions, a direct comparison with conditions in other countries is not particularly useful.’ In other words, diesel specs for Siberia are not relevant to South Africa.
What happens to diesel as freezing occurs?
At low temperatures, long-chain paraffinic chemicals occurring in diesel fuels start to form wax crystals at the Cloud Point which can begin to block fuel systems via the fuel filter. A vehicle experiencing cold operability problems may start satisfactorily but performance deteriorates and the vehicle stops as the engine is starved of fuel. The low temperature performance of different vehicles varies greatly due to design features such as positioning of filters, tanks, routing of pipe-work and additional measures like fuel line and filter heaters, now standard on many passenger cars in Europe.
Table 1 (below) shows an example comparison between South Africa, European and India sub-continent CFPP specifications that reflects the vast difference between climates and countries.
To improve cold filterability and match CFPP standards, fuels are almost always treated with additives. Wax crystals grow as large platelets in untreated fuel that will rapidly restrict and then stop fuel-flow through lines and filters. The additives treated in the correct manner will modify the wax platelets to needles, allowing for operation at colder temperatures for a while, until the diesel heats up. But these additives cannot be added when the problem occurs and don’t come cheap so it’s not just a question of throwing chemicals at the market for an unusually low-temperature weather spike.
Water aggravates freezing
To prevent ice-crystal formations in a fuel system, it is important to ensure that water is regularly drained from fuel line water-traps or sedimentors. This applies to bulk fuel storage and vehicle fuel systems. And while you are removing water from your fuel, make sure that brake reservoirs are also clear of water – too many trucks have excess condensed water in air brake systems. Frozen brake systems are not the way to reduce stopping reaction times!
What other counter measures can be taken?
Minimise diesel waxing problems by parking trucks in a sheltered position or better still, garaging the vehicle - and use the correct grade of fuel for the season.
Also stick to changing fuel filters at recommended intervals.
Once started, the vehicle should be driven so that the diesel can warm up within 20 minutes.
And if biodiesel is being added, make sure it matches the certified spec SANS 1935:2004. Note that SANS 342:2006 is the standard for automotive (retail and commercial) diesel that has not been blended with more than 5% biodiesel.
Loose addition of petrol and illuminating paraffin is also not the way to go – this is asking for trouble on modern engines.
More importantly, do not jump to conclusions and perceptions that drivers and others may generate. Gather facts, as it may not be a fuel problem – or if it is a fuel problem, find out why? Our local diesel fuelwinter standard of -4˚C is adequate to match most conditions but downtime is costly. Don’t be caught frozen!
Reference sources: Petrol & Diesel in South Africa and the impact on air quality – SAPIA book November 2008; BOSCH Automotive handbook.
Copyright © 2010 FleetWatch magazine and FleetWatch On-Line.
No part of this publication may be reproduced without the prior written permission from the publishers.
Views published are not necessarily those of the publishers.