As a car guy I’ve been following the story about Volkswagen Group’s diesel emissions scandal with interest. And especially interesting is what is being left unsaid – that if it weren’t for our obsession with CO2 emissions this scandal may never have occurred.
The bad guys: hydrocarbons, CO, and NOx
First, a little primer on vehicle exhaust emissions and the history of emissions controls. Vehicle internal combustion engines whether gasoline or diesel-fueled produce three main pollutants, hydrocarbons, carbon monoxide (CO), and Oxides of Nitrogen (NOx). Additionally Diesels also tend to produce particulate matter (soot/smoke). These pollutants come from the combustion of a mixture of air and fuel. Hydrocarbons consist of unburned or partially burned fuel. CO is also produced by incomplete combustion of fuel. On the other hand, NOx is created by nitrogen and oxygen combining under high temperature conditions, typically when an engine is running at its most efficient.
Early attempts at reducing emissions focused on ensuring fuel was more completely combusted and at lowering the temperature at which combustion occurred. The methods used were pretty crude and adversely impacted the power, economy, and driveability of the cars to which they were fitted. Using these methods would only reduce emissions so far. Something else was needed. Enter the catalytic converter. This device uses catalysts to promote oxidation of hydrocarbons and CO, and reduction of NOx. Catalysts come in two flavours – two-way catalysts which only oxidize, and three-way catalysts which oxidize and reduce. Just one problem, for a three-way catalyst to work oxygen levels in the exhaust gasses cannot be excessive. In recent years, increasingly sophisticated engine management systems have made cars cleaner and cleaner, but we still can’t get away from that optimum amount of oxygen required for three-way catalytic converters to function.
In theory a certain amount of air is required to completely burn every molecule of fuel. This “perfect” ratio of air:fuel is called the stoichiometric ratio. The stoichiometric ratios of gasoline and diesel are remarkably similar being 14.7:1 for the former and 14.6:1 for the latter. However, due to the unique properties of both gasoline and diesel and what is being asked of them inside an engine, the two types of engines both behave very differently if run with a stoichiometric air:fuel mix. Gasoline is a quick-burning fuel that will burn relatively cleanly at or near stoichiometric air:fuel ratios. Diesel burns much more slowly and requires extra air to speed up the combustion process. If you try to run a diesel engine at a stoichiometric air:fuel ratio, it will produce vast amounts of particulate matter (smoke!) because while there is enough oxygen present (in theory!) to support complete combustion, it can’t do so quickly enough within the brief amount of time available in the engine’s combustion cycle. Therefore diesel engines tend to run at much leaner air:fuel ratios. How much leaner? You can read through this thread to get an idea. Lean-burn gasoline engines have also been manufactured with the benefits of reduced hydrocarbon and CO emissions and improved fuel economy. Unfortunately increasing the ratio of air to fuel means that more unconsumed oxygen is contained in the engine’s exhaust gasses which means that three-way catalytic converters will not function properly. And to make matters worse, the increased temperatures and combustion pressures at which these engines operate can actually increase NOx emissions. If it was just hydrocarbons and CO we were worried about we could make engines cleaner and more fuel efficient simply by optimizing them to run as lean as possible.
What about CO2?
The three pollutants discussed above only constitute a small fraction of what comes out of your car’s exhaust. Nitrogen which for the most part goes through the combustion process without reacting (except for the bits that form NOx) comprises the majority of it, just as it comprises the majority of the air going in. The other two important by products are carbon dioxide (CO2) and water vapour which are products of complete combustion of the fuel. And as we all know, as the alleged source of climate change, CO2 represents the GREATEST THREAT OUR PLANET HAS EVER FACED.
This presents a real dilemma for auto manufacturers like Volkswagen. On the one hand regulators mandate low exhaust pollutant levels, while on the other hand they put pressure on manufacturers (usually through tax penalties) to produce more fuel-efficient vehicles. Unfortunately prioritizing one of these goals almost always has a negative impact on the other. Volkswagen’s solution was to prioritize fuel economy, first by popularizing frugal diesel-engined cars, and when tightening emissions standards caught up with them, by cheating the emissions tests. To give you an idea of how much of a better car the “dirty” diesel is for the average driver, compare the fuel economy/CO2 ratings of the Volkswagen Golf with the smallest available diesel engine versus the smallest gasoline engine . While the PS (German hp) ratings are close, the larger diesel engine will provide far more torque and a better driving experience.
So far,there has been very little discussion in the old-stock media about the trade-off inherent in choosing better air-quality versus lower carbon emissions (the trade-off has been mentioned but no-one actually wants to talk about which should take priority). My advice, if you drive one of the affected Volkswagens is to avoid the factory reprogramming that will shortly be offered, or if you can’t, be prepared to find an independent tuner who will restore the car’s engine management computer back to its original settings (or better – diesels respond well to remapping). If the NOx doesn’t kill us with asthma or cancer the CO2 will through global warming and at least with the former you’ll go to the grave with a bit more money in your pocket.