The Importance of Thermal Management For Modern Diesel Engines

The introduction of very stringent exhaust emissions limits such as the Euro VI and EPA 2015, or Tier 4 Final regulations in most key markets has led to some significant challenges. The legislation seeks to reduce key compounds in the exhaust that are known to be very harmful to human health, these primarily being Nitrogen Oxides (NOx), Hydrocarbons (HC), Carbon Monoxide (CO) and Particulate Matter (PM). NOx and CO in addition to being directly harmful to health have the further negative impact of forming ground level Ozone O3.

The situation in heavy-duty vehicles is different to passenger car as the engines are tested to a more demanding duty cycle than the passenger car engine market. The WHDC [1] test cycle includes more time spent at full load, a highly transient cycle and also a number of cold start tests. Whereas the passenger car testing is currently carried out to the NEDC [2] driving cycle, where NEDC is not a particularly demanding driving cycle. In the future passenger car emissions testing is going to be carried out using the hopefully more representative RDE testing procedures [3]. Several studies have shown that under real world conditions heavy duty vehicles are much more closely aligned with the regulatory emissions limits than passenger cars. Not to mention the current (Dec 2015) controversy surrounding passenger car emissions not meeting the regulatory limits in real world driving conditions. Even not considering this likely cheating, the passenger car and to a lesser extent the heavy duty emissions standards allow real and legitimate possibilities for in use emissions to be significantly different to those required by law due to the very limited area of the engine load map covered by the mandated tests.

So why is good thermal management important to the modern diesel engine ?

The method of choice to keeping diesel exhaust emissions under control is known as Selective Catalytic Reduction or SCR for short. SCR involves spraying a fine mist of Urea (AdBlue) into the hot exhaust stream, which reacts with the NOx in the presence of a catalyst does exhaust trap reducing it to less harmful compounds.

In order for the SCR reaction to work the exhaust gas has to be above a certain temperature, otherwise no reduction will occur. When the exhaust gas temperature is low the heavy-duty engine will have alternative control strategies to reduce the legislated emissions by altering the engine timing and amount of exhaust gas being recalculated back into the engine. There may also be other control actions involving creating elevated exhaust back pressure with a control flap or other device to raise the exhaust temperature, in some cases fuel is even burnt directly in the exhaust to raise the system temperature. All of these methods have a negative impact on fuel consumption, and typically are not as effective at reducing NOx emissions as the SCR system. So they have undesirable side effects that should be avoided.

There is another factor, these modern engines will also typically have an exhaust particulate filter (DPF) in addition to the SCR trap. Both the particulate filter and the SCR trap sit in the exhaust stream and get blocked by excessive soot and particulates coming from the engine. The engine control strategy to remove the blockage is known as a regeneration, and involves getting the DPF and SCR really hot to clean out and regenerate them. Eventually the DPF can become so blocked that it can only be cleaned by removal from the vehicle. Again a set of undesirable side effects from the vehicle users point of view.

So maintaining the engine at the correct temperature to help optimise the exhaust gas temperature is now even more critical than before. A cold running engine will have a negative impact on both fuel consumption, exhaust emissions and result in more frequent DPF blockage and cleaning.

AVID’s eFan Micro Hybrid System is proving to be a great benefit to these modern low emission engines. AVID’s electric fan and proprietary control system technology helps to ensure the engine reaches the correct operating temperature more quickly than conventional technology and remain there even under highly transient driving conditions. This is not possible with conventional heavy duty engine cooling systems, even those with controlled hydraulic or magnetic clutch systems due to the inability to manage fan speed independently of engine speed and target the required cooling air flow at only the places it is needed controlling the engine coolant temperature and charge air temperatures independently and with a multi variable control algorithm.

The electrification of the main engine coolant pump also provides an opportunity to improve engine thermal management. This approach has long been known to deliver an efficiency improvement, but the benefits are minimal at <1% fuel consumption on the NEDC cycle. However AVID has repeatedly been able to show that under real driving conditions the benefits are much greater, add to this the possibility to further improve engine warm up and operating temperature control and minimise the time spent on cold maps the benefits with the new testing regimes are clear. AVID can supply custom electrified pump solutions at 12V, 24V and 48V.

Liquid or indirect cooled Charge Air Cooling systems (LCCAC, WCCAC) require an electric coolant pump and a secondary low temperature cooling circuit. These systems are finding favour due to the ability to improve engine transient response due to the possibility to decrease the volume of the engine air inlet system, and more accurately control the charge air temperature than a conventional air to air system. Improving transient response can be beneficial simply to offset the negative impact of the cold operation engine map.

It should also be noted that this is not a specific problem of Diesel the fuel, as has been reported in the press, but more the Diesel engine cycle which burns the fuel under high pressure and temperatures, it is at these high temperatures and pressures that the harmful compounds are created but unfortunately this is also what makes the Diesel engine efficient. Modern high compression ratio, turbo charged direct injection gasoline engines also face challenges with exhaust emissions of a similar nature, and for the same reasons.

[1] WHDC Testing

[2] NEDC Testing

[3] RDE Testing

[4] Summary of SCR process