Homebrew Optical Sensor Helps Your Diesel Pass The Smoke Test

We’ve all heard of the smoke test, and we know that it’s the lowest possible bar for performance of an electronic device. If it doesn’t burst into flames when power is applied, you’re good to go for more functional testing. But the smoke test means something else for cars, especially those powered by diesel fuel. And passing diesel exhaust tests can become something of a chore.

To make passing these tests a little easier, [Janis Alnis] came up with this diesel exhaust monitor that measures the opacity of his car’s emissions. The sensor itself is quite simple, and mimics what commercial exhaust analyzers use: a LED and a photodiode at opposite ends of a tube of a specified length. Soot particles in exhaust passing through the tube will scatter light in a predictable way, and the numbers work out that a passing grade is anything greater than 53% transmission.

The sensor body is cobbled together from brass pipe fittings with glass windows epoxied into each end. Exhaust enters via a tee fitting attached to a hose and sampling tube, and exits through another tee. One window of the sensor has a cheap battery-powered flashlight as a light source, while the other end has a Texas Instruments OPT101 photodiode sensor. The sensor is connected to one of the analog inputs of an Arduino, which also runs a 128×64 pixel LCD display — inspired by this air quality meter — to show the current smokiness both graphically and as a percentage. The video below shows the sensor at work.

While there were some issues with soot buildup and water vapor condensation, using the sensor [Janis] discovered that a little bit of a warm-up drive got things hot enough to clear up his ride’s tendency to smoke a bit, allowing him to pass his inspection.

50 thoughts on “Homebrew Optical Sensor Helps Your Diesel Pass The Smoke Test

      1. Keep in mind that many diesel exhaust particulates can fall to the ground, and not need with air. I was disgusted at the amount of soot in public (such as handrails) in some European cities. No wonder they all wear black.

          1. Does it pass the smell test? Brake material on a set of pads is what, ~1KG? You need to change them every 50-100K km. Very dirty diesel will produce 6g of crap per Liter, modern Ram 1500 Diesel does 1 g/L. You end up between 1 and 6 tanks of fuel to emit as much crap as a complete set of pads.

          2. https://www.greencarreports.com/news/1127424_study-particulate-emissions-from-tire-wear-is-higher-than-from-tailpipes

            >a “popular family hatchback” with new, correctly-inflated tires. The hatchback’s tires emitted 5.8 grams of particulate matter per kilometer, compared to 4.5 milligrams per kilometer from the exhaust, according to Emissions Analytics.

            The emissions also include that which is abraded and picked up from the road and thrown in the air by the tires. If that was all coming off of the tires then they would be gone after a thousand kilometers.

          3. >Used tyres produced 36 milligrams of particles each kilometre, 1,850 times higher than the 0.02 mg/km average from the exhausts. A very aggressive – though legal – driving style sent particle emissions soaring, to 5,760 mg/km.

            Ah, so the 5.8 grams per km in the previous article was from that. Sloppy writing from green car reports as usual.

          4. Dude: That person wasn’t really trying.

            My toy has almost as much rubber in the wheel wells as on the tires. Fun!

            Traction control is for pussies. (Kegel muscles superior traction control method.)

      2. The test conditions are basically redlining the engine at 5,000 RPM, which will pass soot through if the catalytic converter is all clogged up. Diesel engines don’t work efficiently at high revs because there’s not enough time for the diesel droplets to burn. The reason why the converter is there is exactly to catch the soot and the gradually burn it away as you drive along, which doesn’t work well in city driving conditions and the engine needs to run a filter cleaning cycle more often – which typically involves shooting extra fuel and air in the exhaust to heat up the catalytic converter until it’s clear. Not all cars, especially older cars, will do this. The car’s computer will tell you to keep on driving as it does so, and if you stop in the middle, well, the filters will not get cleaned and the problem gets worse. This is a common annoyance for diesel drivers – they have to run “victory laps” every few weeks when the computer says so if they drive short distance only.

        That was the problem – not getting the engine hot enough for long enough, often enough to keep the filters in working condition. If the car was used in a more mixed setting, the filters would work and the exhaust would stay clean. The solution was periodically running a couple hundred km on the highway to do what the exhaust treatment system wasn’t doing.

        The engine at idle will produce 2% transmission loss as noted by the author.

        1. You couldn’t be more uneducated about the diesel cycle and associated emissions equipment…
          That is not why the catalytic converter is there because then what task would be left for the diesel particulate filter?

          1. The diesel particulate filter in modern diesel engines IS a catalytic converter.

            The normal catalytic converter in a gasoline engine burns away incompletely combusted HC and CO molecules in the gas phase – it’s less effective for solid soot particles, so diesel cars have an extra filter. With older engines, this is a consumable item which needs to be replaced periodically. In newer engines the particulate filter is either a second passive catalytic converter, or an active catalytic converter where the ECU will periodically run a cleaning cycle on top of being passively catalytic.

          2. https://en.wikipedia.org/wiki/Diesel_particulate_filter

            >The regeneration process occurs at road speeds higher than can generally be attained on city streets; vehicles driven exclusively at low speeds in urban traffic can require periodic trips at higher speeds to clean out the DPF.[5] If the driver ignores the warning light and waits too long to operate the vehicle above 60 km/h (40 mph), the DPF may not regenerate properly (…) Some newer diesel engines, namely those installed in combination vehicles, can also perform what is called a Parked Regeneration, where the engine increases RPM to around 1400 while parked


            >The Engine Control Unit (ECU) will make adjustments to the engine RPM’s and diesel fuel injection, either through fuel injection directly through the exhaust system into the DPF, or changing the EGR valve, injection timing, or intake air flow, resulting in higher temperatures (600 degrees C) in order to burn off soot buildup.

          3. The filter ate my links to source articles, but basically wikipedia and other sites point out that there’s three major styles of DPF: plain filter, passive catalytic, and couple different styles of active catalytic filters which vary the engine fuel map, EGR, valve timing, injection timing, to make the exhaust hotter and/or add oxygen to the exhaust which is then used to burn fuel that is injected before the DPF to regenerate it. Some electrically heated DPFs also exist.

          1. What he’s probably talking about is fouling the filter by generating a spot of soot on the filter that doesn’t clear up because oxygen can’t reach the catalyst and more soot keeps building up, partially clogging the filter.

            This also happens when the driver ignores and interrupts the regeneration cycle, but the ECU will typically throw up all sorts of warnings and may put the car in limp-home mode.

        2. I drive a diesel vehicle and you have no idea what you’re talking about….

          DPF regneration happens any time the car is over about 40-45mph for more than a few minutes; almost any highway driving rapidly regenerates the DPF.

          If you drive long enough at low speeds and part throttle, you might eventually notice the transmission is holding a gear longer than it otherwise would.

          If that still doesn’t cure the problem, then you get a code and warning light. Then you need to drive at 40+mph for 10-15 minutes, yes. If that still doesn’t work, then you use a scan-tool to trigger the DPF regeneration mode where it maintains a high idle.

          In over 100,000 miles of driving I’ve never seen the warning light come on.

          Also you don’t need to “drive a couple hundred km”, you can on most cars remove the DPF and back-flush it with an electric power washer and re-install, good as new.

          1. >DPF regneration happens any time the car is over about 40-45mph

            Yes. Passive regeneration. It’s a catalytic converter – any time it gets hot enough it starts to burn off the accumulated soot.

            >then you use a scan-tool to trigger the DPF regeneration mode

            I’ve never needed that. The car’s computer just says “Keep driving. Regenerating filter”, and it does so automatically once it determines the filter needs regenerating.

            >you can on most cars remove the DPF and back-flush it with an electric power washer

            YMMV. I wouldn’t recommend.

            Remember these are small diesel cars we’re talking about, tiny turbo diesels in tiny European hatchbacks. If you take out the DPF you’ll probably void the warranty and cause yourself a 2,000EUR repair bill.

        1. It does. That’s why it appears colored, and the surface also scatters light so not all of it ends up towards the other end of the tube.

          Regular air also does the same thing to some extent. Water, CO2, absorb and scatter particular wavelengths. The light source is not perfectly collimated (not a laser beam) so it spreads over distance and again some of the light takes a longer path by multiple reflections (which adds absorption and scattering…) etc. etc.

          There’s also the question whether the tube is supposed to direct light down the tube by reflection, which changes when the tube gets dirty, or whether it’s supposed to absorb any light that isn’t going straight down the tube to eliminate that error.

          Either way, what you put into the tube doesn’t all come out the other end even under clean air conditions.

    1. New diesel engines (anything built in the last 20y or so) are subject to an entirely different standard. The 53% number is a transient maximum in the snap test. It’s only for older engines. (Pre 1990s) It is not a steady state measurement. Basically the engine is warmed up (180F+ – operating temp), sensor installed, and they punch the throttle. It’s a near worst case test scenario.

      Federally 1991 and newer MY engines must be 40% or less. DPF equipped engines can’t hit that, anything over a few % is a total failure of the DPF.

      Multiple states (CA, NJ to name just two) implement far stricter standards for the snap test.

    1. Rather to figure out if injectors are worn and are pissing in fuel instead of correctly dosing it.
      Or if some absolute knuckledragger thought that applying the same type of “tuning” techniques used on old mechanical pump tractors for tractor pulling competitions are also a great idea for modern road going diesels.

      1. Those wouldn’t change much after warm-up though.

        I’m refering to this specific part: “[Janis] discovered that a little bit of a warm-up drive got things hot enough to clear up his ride’s tendency to smoke a bit”

        It’s like…no shit, Sherlock. But then again, there’s so many diesels used for trips that they’re just utterly unfit for.

        1. Yeah, you’re right. Always use the right vehicle. I keep three vehicles in my pants pockets at all times. A diesel for long trips, a gasoline powered one for shorter trips, and an electric vehicle for tooling around the neighborhood to buy groceries. Not.

          If you’ve got a car, you use it for what you have to do. Very few people have the luxury of being able to choose what type of engine to use for a particular errand.

          1. Problem is, at least here in Europe, that Diesel (as in fuel) was heavily subsidized until about two years ago.

            Think of it this way: Your total daily commute is 30km tops, what’s usually considered short trips. You want a car for that. Do you buy the gasoline one with lower mileage and higher fuel cost or do you buy the subsidized diesel with higher mileage and lower fuel cost?

            Too many people looked at it this way and ignored all the drawbacks and issues of a diesel on short trips. It uses less fuel and the fuel is cheaper, it would be illogical for them to buy a gasoline one, wouldn’t it? But on the other hand they get: Constantly clogged DPF, rusting exhaust, higher wear in the engine, emission control basically never working, ungodly amounts of soot being everywhere, diesel scandals resulting in them not being allowed to go into cities/regions and so on and so forth.

            Even if you have just one vehicle, you can have one that fits the majority of your actual needs instead of just being cheap at the gas station and/or highly practical for that one time in a year you’ll do an actual long trip.

          2. Lol I have a 1 ton truck a 1/2 ton HD truck, a 1998 forester and a 2014 spark. Not that hard to have the right tools for the job if it’s actually a job. Most people drive a truck around here to Walmart and I’m going to change irrigation on 3 10 acre fields in my EV. But but big truck for farm bois or else?!?!?

          3. More like, diesel wasn’t subsidized but gasoline was and still is extremely heavily taxed. 60-80% of the fuel price is commonly tax, whereas diesel enjoys lower tax because it’s used for business.

          4. Before some smart-ass complains: taxing something less isn’t a subsidy.

            A tax break is a subsidy, but that would require that both gasoline and diesel are first taxed the same under one tax rule, and then some people/uses/cases are given a reduction or an exemption. That also happens for some diesel users, but by default gasoline and diesel aren’t taxed the same to begin with – gasoline was just given a higher tax in attempts to curb gasoline imports, while diesel was taxed less because it hurts private business.

            In the 70’s before diesel engines were widely available for regular cars, gasoline went short in supply and people started driving on all sorts of ersatz fuels and engines modified to use them, such as kerosene and turpentine. Car companies started introducing diesel models to make use of the fact that diesel was still plenty available and cheap for the farming and general transportation industries, and the lower taxation meant that they stuck around.

          5. @Dude: This is where you’re wrong (Sounds generic, but I’ve read some other comments of yours in here, and they’re mostly incorrect aswell).

            For something to be subsidized it does not need to have been equally priced/taxed before. It is possible to subsidize things from the get-go. In germany for instance diesel was incredibly cheaper than gasoline. And there was no difference whether you’re a business or a consumer, it just was cheaper, because of higher efficiency and less CO2. As I’ve elaborated above, everyone and their nan started buying diesels and still is today. But then we started looking at more than just CO2 and realized that diesel exhaust contains a shitton of neurotoxins and microparticles. Add some fraudulent activity, a diesel scandal and suddenly the current lax taxation of diesel is being questioned.

  1. Domestic fire detectors that use optical detection work a bit differently. Rather than measuring transmission, they measure scatter. The detector is set at right angles to the path of the IR light. In normal situations, no IR light makes it to the sensor. When there is smoke, some light scatters off the smoke particles onto the sensor. You get much better precision from this arrangement than the straight-through version.

  2. > mimics what commercial exhaust analyzers use

    in US. Measuring transmissivity of exhaust gasses is US thing. Civilized world measures O2, CO(H2 compensated), NO and NO2.

    1. Germany has strict exhaust checks.

      Here’s what the German Wikipedia entry says:

      “Fahrzeuge mit Dieselmotoren (mit und ohne OBD)

      Bei Dieselfahrzeugen ist einzig der Trübungswert (k-Wert) des Abgases durch Ruß entscheidend. Der gesetzliche Grenzwert liegt bei 2,5 m−1. Bei Fahrzeugen nach Euro-4-Norm gilt ein Grenzwert von 1,5 m−1.

      Quelle 41. VO AU-Richtlinie ”

      English translation:

      “Vehicles with diesel engines (with and without OBD)

      In the case of diesel vehicles, only the turbidity value (k value) of the exhaust gas due to soot is decisive. The legal limit is 2.5 m−1. A limit of 1.5 m−1 applies to vehicles in accordance with the Euro 4 standard.

      Source 41. VO AU Directive”

      The standards for gasoline engines refer to the gases you listed, but for a diesel only the transmissivity matters.



    1. It’s normally done backwards, like oxygen sensing, so if there’s free oxygen then there’s no hydrocarbon unburned… they figure. But lotta O2 means Nitrogen oxides must be happening, so in lieu of actual control, make it play ping pong across the lambda point.

      1. Really , proper mot style gas analysers that I’ve worked on or used have the ability to measure lambda(inferred from O2) O2(obviously), nox , HC,co and co2. And with the data you can make an educated guess if HC is high due to fueling or burning oil for example. This implied HC is directly measured and not inferred like lambda.

        And here in the UK diesels are only tested on opacity.

        1. Of course that’s assuming the engine is running at the proper temperature. With a cold engine and a cold catalytic converter, excess O2 and HC can both happen at the same time.

          This is a problem with gasoline vehicles as well, because there’s ethanol in the fuel, and it converts into acetaldehyde when the engine is started rich, up until everything reaches proper temperature (you can smell a kind of fruity odor). The car burps up a bunch of carcinogenic irritants that cause asthma and lung cancer, and smog, and that happens millions and millions of times a day in a large city with short stop & go driving.

  3. Our Hyundai Highlander has a turbo diesel and runs so clean that you can rub the inside of the exhaust pipe with your finger without picking up any soot. It doesn’t even smell like a diesel powered vehicle when it is running. I guess that just demonstrates how important the engineering and maintenance of the vehicles is. If these sensors can be used to fine tune an engine to that extent then this is a very useful hack indeed.

  4. I am surprised that a “flashlight” would be used instead of an LED on a well regulated current. Power weakens as the battery dies and then there is that ever so often flashing and dimming that is ubiquitous with said lighting device that contains throwaway cells. Even my 18650 light occasionally winks and needs a rap, at least the switch is electronic not another dirty metal contact in the path.

    Recently I made a rare trip to the mall on a friday evening for something specific and was surprised by all the adult-kids blowing smoke from their tricked up diesel trucks.

    They can do this light transmission thing across uphill ramps and other one lane throttle-open “smoke traps”! Add a plate read and you get a ticket in the mail.

  5. I am a heavy duty diesel tech. The dpf is not a catalyst. There are 3 parts to a modern diesel aftertreatment. First is the DOC (diesel oxidation catalyst). That is a catalyst and the purpose is to use fuel injected into the exhaust (or intentionally overfueled by the injectors) to heat exhaust temps to over 1000F. The second part is the DPF (diesel particulate filter) who’s only job is to trap particulate matter. The third part is the SCR (selection catalyst reduction) who’s job is to react with the DEF (diesel exhaust fluid) to reduce nox emissions. During a regen the doc heats up the dpf hot enough so that the soot (particulate matter) is turned into ash. Which is much smaller then the soot so you can fit more of it in the dpf. Eventually the dpf fills up with ash and has to be cleaned out or replaced. This is a very basic explanation but I hope it clarifies things for you guys/girls.

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