I’ve been wondering if modern cars can actually adjust for more, or less, energy dense fuels. I understand that ‘Octane’ does not exactly measure the same thing.
In the old days, if you wanted to adjust the richness of your mixture, you had to adjust the carburetor. Then came polution control and lots of new technology. Now I’m reading about the VW scandal, and how they are now adjusting the air-flow measurement technology, so that they can use a more accurate ‘map’ – sounds like they are back to ‘adjusting the carburetor’ to control fuel mixture.
So, what’s the state of technology? Does an average American modern gasoline engine measure exhaust hydrocarbons or NOx and adjust the mixture?
No. They actually measure oxygen and fuel going into the engine, oxygen coming out of the cylinders, and oxygen remaining after the catalytic converter. From these inputs, the cars computer can infer whether it is burning the right amount of air and fuel and whether everything is running right.
No, it measures the oxygen in the exhaust and adjusts the mixture. The map is a precalculated multi-dimensional graph of various inputs (exhaust O2, engine temp, throttle position, etc., etc.) and associated appropriate mixtures.
Does your state/country require periodic smog checks? Even see the machines they use to measure those gases? It would cost $$$ to add that kind of monitoring, and as noted already, they really don’t need it.
there’s no “exactly,” all a fuel’s octane rating does is tell you its relative resistance to autoignition.
No, that’s not how any of that works. VW’s problems were with diesel (compression ignition) engines, which operate rather differently than gasoline (spark ignition) engines.
in a nutshell: spark ignition engines rely on a spark plug to ignite a “homogeneous charge” (fuel thoroughly mixed with air) mixture. If the mixture is too rich (excess fuel) or too lean (insufficient fuel) it won’t ignite via spark. Modern gasoline engines- as Gary T says- monitor the oxygen content of the exhaust stream via O2 (or EGO) sensors. Gas engines use a relatively simple three way catalyst; inside the catalytic converter are three elements. 2 are oxidation catalysts to oxidize (burn/add oxygen to) excess hydrocarbons into H2O and CO2, and carbon monoxide into CO2. the third is a reduction catalyst which reduces (strips oxygen from) nitrogen oxides (NOx) like NO2, NO3, etc and turns them into gaseous N2 and O2. Oxidation catalysts work best when there’s an excess of free oxygen in the exhaust, reduction catalysts work best when the exhaust stream is oxygen-poor. So, modern engines rapidly oscillate between slightly rich and slightly lean to keep all three catalysts working efficiently.
diesel engines don’t rely on a spark to ignite a homogeneous charge. They’re stratified-charge engines, and the fuel begins burning in the hot compressed air as it’s being injected. Which means a couple of things: 1) the high compression/chamber temperatures generate a lot of NOx, and 2) the short time the fuel has to evaporate and mix with the air in the chamber means there are “pockets” of localized rich zones which create particles of solid carbon (soot.) To meet current emissions standards, modern diesels need a number of different emissions controls:
a high amount of exhaust gas recirculation (EGR) to reduce the amount of NOx the engine generates,
catalysts to oxidize any unburned hydrocarbons leaving the cylinders,
a catalyst to deal with the remaining NOx, and
a diesel particulate filter (DPF) to trap and hold soot particles until they can be burned off.
#2 is easy, it’s still a simple oxidation catalyst like gas engines use. Diesels- by way of working over a huge range of air-fuel ratios- have plenty of free oxygen in their exhaust, so oxidation catalysts are easy.
#3 is hell. Diesels almost never run rich. since they always have a lot of free oxygen in the exhaust stream, a simple rhodium reduction catalyst won’t work. it’ll just pull free oxygen instead of stripping it off of NOx. So, the industry solution was to use a Diesel Selective Reduction Catalyst, which uses injection of a fluid containing urea which then breaks down into ammonia and allows the catalyst to get rid of nitrogen oxides.
Volkswagen’s malfeasance came about because Ferry Piëch handed down a mandate that VW would be the “#1 automaker in the world.” Which meant they were chasing sales volume, and were trying to compete with everyone else on price. They were sharply criticized for the de-contented, cheap 2011 Jetta the US got.
Also, they had a mandate to market “Clean diesel.” the problem was that they couldn’t meet the US emissions standards at the price point VW management demanded the cars meet. so they devised schemes where the cars would pass if they knew they were being tested, but spew junk all other times. The 2009-2014 cars didn’t have a selective reduction catalyst, what they had was called a “lean NOx trap.” supposedly it would adsorb NOx inside the DPF until it could run a regeneration cycle. Unfortunately, under normal operation the engine produced far too much NOx for the trap to handle, so in order to pass they had to alter the engine’s operation under test to stop it from generating too much NOx.
nope, as has been said they only measure the oxygen content of the exhaust.
diesel engines, on the other hand, do have NOx sensors to monitor the efficiency of the selective reduction catalyst.
Just a minor supplement to JZ’s comprehensive answer:
The OP asked about hydrocarbons vs NOx in exhaust. When a spark plug in your engine fires and burns the fuel with oxygen from the air, you either have more air than you need, more fuel than you need, or exactly the right amount of both. For gasoline, “exactly the right amount of both” means an air:fuel ratio of about 14.7:1.
If you have more fuel than you have air (running rich), then you’ll expel some unburned hydrocarbons in your exhaust. If you have more air than you need for the amount of fuel in the cylinder (running lean), the nitrogen in the air will tend to grab the extra oxygen and you’ll be spewing NOx. JZ mentioned that diesel engines tend to run lean, which is why NOx emissions are such a problem for them.
A fuel/air mixture with exactly the right ratio of air to fuel is called a stoichiometric mixture. Both stoichiometric mixtures tend to burn hot (there’s no extra fuel and no extra air to absorb the heat of combustion) so to avoid engine damage, the car’s computer (ECU) will typically only try to apply a stoichiometric air/fuel ratio under light load. When you floor the throttle, you’re running rich.
OP, you also asked whether ECUs can adjust for fuel energy density. The answer is basically “yes,” but fuel energy density doesn’t vary a whole lot. JZ is absolutely right that octane is unrelated to energy content. The main variance in gasoline energy density (in the US) comes from adding ethanol to gasoline. In many parts of the US, all octane grades have about 10% ethanol added. That reduces the energy content of the fuel.
Gasoline has about 34 megaJoules per liter, while ethanol has about 20 MJ/l. That means that a liter of gasoline cut with 10% methanol (by volume) has about 32.6 MJ/l, or about 4% less than straight gasoline. (Interestingly, adding ethanol to gasoline lowers its energy content while raising its octane rating).
Your car doesn’t know the energy content of its fuel. When operating in closed-loop mode, if your oxygen sensor tells the ECU there’s extra oxygen in your exhaust, the ECU adds fuel or restricts the air intake (at the throttle body). Gas with ethanol needs a little less air than pure gasoline, but your car just reduces the air content until the O2 sensor doesn’t smell oxygen. This is a bit of a simplification, of course.
You’ll notice that the stoichiometric ratio for air and gasoline is 14.7:1, and atmospheric pressure at sea level is 14.7 pounds per square inch. This is not a coincidence.
No, that’s not true. It’s completely coincidental.
Thank you all. In my head I knew the answer must be known to educated people, but in my heart I couldn’t believe anybody could know an answer I didn’t already know… My knowledge of vehicle intrumentation stopped in the early 70’s, when it was still a research project.
Due to our weather patterns, we don’t have a smog problem anywhere in Aus, and polution control is not much of a local issue. There certainly is no regular testing of ordinary cars, and we tend to just follow overseas regulation after it becomes mainstream.
Yes, modern cars have sophisticated monitoring systems that allow automatic adjustment (within limits) of fuels with different energy content and octane ratings. The fuel injection system knows the amount of fuel added - per cylinder in some designs. The exhaust O2 sensor helps indicate combustion status.
They use engine “knock” sensors to detect incipient knock, and will automatically adjust engine parameters to avoid this, including turbo boost, fueling and ignition timing: Engine knocking - Wikipedia
There are two types of knock sensors: acoustic and ion flow (aka ion sensing). Some cars have a separate acoustic knock detector on each cylinder. Ion flow runs a current through the spark plug electrode after the ignition event to sense characteristics of the combustion gas. It can tell much more about combustion behavior than acoustic detection. In theory ion sensing can detect or infer emissions data to some degree: Making sense of ion sense technology | Vehicle Service Pros
Technologies like these have enabled production gasoline vehicles to reach 14:1 compression ratio, as seen on the new Mazda engines:
The adaptation range to accommodate differing fuel octanes and energies will vary with each car and engine control system. Flex fuel vehicles can run on varying fuel blends (with varying energy densities) ranging from 100% gasoline to 85% ethanol and 15% gasoline. I think they automatically detect this and make the necessary changes to engine operating parameters. E85 has only about 73% of the energy per gallon as gasoline, so the fueling and engine control system must adapt over a pretty wide range.
