December/January 2022

BY Frank Massey W ith the COP 26 global environment talks recently taking place in Glasgow, I thought I would revisit how diesel vehicles perform with a much more technical rather than political approach. Please refer to Fig.1. To begin, let us look at the fuel itself. It must be compressed to burn more efficiently. In fact, most of the more recent reductions in diesel emissions are the result of improvements in compression efficiency and injector technology. Diesel burns slowly and therefore needs to be delivered in a highly atomised format as possible if efficient combustion is to be achieved. I deliberately avoided the term ‘complete combustion’ as this is not possible. It must be delivered into the cylinder progressively across the combustion cycle with quantity and timing courtesy of the microchip. Effects Please refer to Fig.2. Next, we need to consider the effect on the engine itself, as this is often overlooked within our industry, especially with regard to servicing and repair, unlike petrol, and that presents its own unique problems. Diesel engines suffer from oily pre and post combustion deposits. It is a costly process to strip and remove deposits 30 AFTERMARKET DECEMBER/JANUARY 2022 TECHNICAL www.aftermarketonline.net PART ONE DIESEL EMISSIONS The technical specifics of diesel, its role and its future are considered by Frank Massey in the first of a two-part series from the intake system, EGR and turbo. I am sure you understand all the problems we encounter on diesel vehicles are related to the emission reduction systems. That very statement confirms how inefficient diesel combustion really is, wouldn’t you say? One of the most obvious steps we could take is reducing the service intervals based not on time or distance but the operating environment. A vehicle driven in urban conditions over short distances will not reach the temperatures required for best combustion performance. Minimum exhaust gas temperatures of more than 200°C, ideal for passive soot reduction, are rarely achieved. Bosch has developed 200°C technology for urban drive cycles. Unfortunately, as I see it, this development uses a close- coupled combined DPF/SCR with a second catalyst further down the exhaust tract. The exhaust gas system is also thermally insulated to maintain internal temperatures. There is a considerable advantage in using premium high-performance fuels offered by leading brands, however there seems to be a fine line in the balance between the advantages and disadvantages with B5-B20 bio-diesel. I am personally not convinced. Lower combustion temperatures can lead to injector fouling and small increases in NOx, and often deliver a small reduction in power output, especially in less-than-ideal conditions. This