Diesel emissions

Part one

By Frank Massey |

Published:  14 January, 2022

The technical specifics of diesel, its role and its future are considered by Frank Massey in the first of a two-part series

With 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.

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 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 can also lead to fuel filter restriction and sump oil contamination, requiring a shorter servicing regime. Significantly, it is not universally approved by all manufacturers either, especially with regards to older vehicles.

In my opinion, vehicles constantly used within this environment i.e., 250-500 miles a month, and that includes a high number of diesel owners, would benefit from flushing and an oil service every three to six months. That statement may sound ridiculous today, but it will not tomorrow. I have often argued that many vehicle owners cannot afford to maintain their cars correctly, adding to an already problematic fuel system.
In addition, many budget service parts on offer do not meet the design criteria set out by the vehicle manufacturers. Air, oil and DPF filtration directly affects the particulates so often discussed by environmentalists. Diesel engines, by nature and design, operate with an excess air ratio. I often wonder how this is affected by inefficient filtration and intake system contamination. Have you ever considered the impact on volumetric efficiency from the oil contamination in the intercooler? Or the reduction in measured air mass from air mass meter contamination? Both of these are critical to SCR reduction calculations.
When conducting a repair or basic service do you check for DTCs, or more importantly, deviation or correction values? These are often stored without a DTC as the required threshold has not been reached.

Part two of this topic will look in detail at catalytic and additive reduction systems which rely on accurate calculation of clean air intake, EGR, temperature, and many other input values.
Gasoline and diesel engines now share the same technological innovations, seeking ever better power delivery with reduced emissions. Extremely high compression ratios demand the ability to alter the valve open and closed periods either side of TDC in order to improve volumetric efficiency and reduce pumping losses. For example, by opening the intake valve early during the exhaust, stroke will improve internal EGR efficiency, coupled with EGR cooling further reducing NOx.

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