November 30, 2020 § 2 Comments
A recent New York Time story discusses an EPA report of diesel pickup truck owners disabling emission controls on their vehicles. The scale of the deception is reported as being the emissions equivalent of putting nine million extra trucks on the road. There appears to be a cottage industry of small companies devising schemes to manipulate the emission controls systems. The term cottage industry likely does not do it justice when one realizes that, according to the EPA, since 2009 more than half a million trucks have been involved. Yet, each of the providers is usually a small player. In 2019 the EPA acted against 48 companies, bringing their decadal total to 248.
The motivation for these actions is not to despoil the environment. It is to improve fuel efficiency and/or engine performance such as (higher) torque. Diesel vehicles are singularly (dis)credited with high production of NOx and particulate matter (compared to gasoline vehicles). In the case of particulate matter (PM), this comparison is strictly valid only when the measure is micrograms per cubic meter and not particle count. But that is a hair to be split another time. Mass based measurement is the only regulatory approach at present.
PM from diesel engines is captured in Diesel Particulate Filters (DPF). After a couple of hundred kilometers of operation, the filter starts clogging. This causes back pressure on the engine and reduces engine performance. Engine manufacturers set back pressure limits at which the filter must be regenerated. This is accomplished by running the engine hot, which produces more NO2, which catalytically oxidizes the carbon deposits. But if the engine duty cycle simply does not achieve these temperatures, the filter will remain clogged. Sometime after, it will cease to function and will need to be replaced. This is a clear example where circumventing the DPF benefits the consumer in performance and avoidance of the cost of premature replacement of the DPF. Enter small player who replaces the DPF with a straight pipe for a fee.
NOx is short for some combination of NO2 and NO. It is harmful to human health directly, and indirectly through ozone formation. Ozone is formed when NOx reacts with organic molecules and sunlight in the atmosphere. Ozone, often erroneously equated to smog, is a component of smog and injurious to health, with children especially vulnerable. NOx is produced when nitrogen in the combustion air is oxidized. Higher engine temperatures create more NOx. The high temperatures needed to regenerate DPFs have this undesirable effect. This is an example of the balancing act required in emission controls. One remedy for this particular trade-off, present in some automobiles, is an auxiliary heating element in the DPF, which fires up at appropriate intervals.
NOx control is addressed in one of two ways. The more foolproof one is Selective Catalytic Reduction (SCR), in which urea is injected into the exhaust. Urea decomposes into ammonia (in two possible steps), which catalytically reacts with NOx to reduce it to nitrogen. Engine performance is unaffected. But the urea canister must be replaced periodically, which consumers see as a nuisance. The system occupies space, so this tends to be in bigger engines (over 3 L displacement).
The alternative, more targeted to smaller engines, is the Lean NOx Trap. Here, the NOx is captured by adsorption onto a surface. Desorption (removal from the surface) is achieved by chemical reduction by exhaust gas components such as uncombusted diesel, carbon monoxide and hydrogen. In an efficiently running engine, these will, by design, be in short supply. Accordingly, the control system deliberately causes the engine to run rich (oxygen starved) to produce these reducing gases. While this achieves the purpose of reactivating the NOx capture, during these intervals, the engine runs with lower power and is fuel inefficient. The fuel efficiency reduction overall is in the vicinity of 3 – 5 %. As a frame of reference, this is the reduction gasoline engines see by the addition of 10 – 15% ethanol in the gasoline, and there is no consumer push back, perhaps because it is not accompanied by a performance reduction. Yet, in diesel pickup trucks, it appears some combination of that and loss of torque is responsible for the public buying avoidance schemes.
Avoidance schemes are known in the industry as “defeat devices”. The practice is so rife, it has a name. Vehicle makers have in the past cut these corners, the most infamous being the VW cheating episode. But this piece is about aftermarket interventions, which are harder to corral, in part due to the sheer number of companies involved. This proliferation is due to the ease of devising the defeat devices. While every state has somewhat different methods for emissions testing, a common one is to simply query the onboard computer for clues on functioning of the emissions system. Altering the onboard programs is enough to produce the avoidance. In other cases, the emissions are tested directly while on a chassis dynamometer (think stationary bike). With such testing, the avoidance must be more sophisticated, as it was in the VW case. Then there is the simple substitution of the SCR device with a tube. A mechanic with a reasonable modicum of competence could execute this.
The defeat devices cottage industry would not exist were there not a market for the product. The pickup truck owners simply want the performance* and may well consider the infraction to be minor. Curiously, they would not be too wrong on the DPF avoidance. The regeneration step described above is combustion of the filter cake and does produce carbon particles. However, the general public is unlikely to be aware of the intricacies of operation of the DPF and the impressions are probably grounded in the beliefs of the truck owner community.
Although more than likely aware of the illegality, truck owners are probably aware that individuals are never targeted by the federal government. In many ways, this is more pain-free than a speeding infraction.
*I can’t get no satisfaction from “(I can’t get no) Satisfaction” by the Rolling Stones (1965), written by M. Jagger and K. Richards
November 30, 2020