Car exhaust contains both carbon dioxide and carbon monoxide, but carbon dioxide (COâ‚‚) is by far the larger component. In a properly running gasoline engine, COâ‚‚ makes up roughly 12 to 15 percent of the exhaust stream by volume, while carbon monoxide (CO) typically accounts for well under 1 percent under normal warmed-up conditions. The two gases come from the same process but tell very different stories: COâ‚‚ is the expected, “complete” product of burning fuel, while CO is an unwanted byproduct of incomplete combustion that signals something went less than perfectly inside the engine.
Why Burning Fuel Produces Both Gases
Gasoline and diesel are hydrocarbons, molecules built from carbon and hydrogen atoms. When fuel burns with plenty of oxygen, the carbon atoms bond with two oxygen atoms each, producing COâ‚‚ and releasing energy. That is complete combustion. In a perfect engine running a perfectly mixed charge, every carbon atom would leave the tailpipe as COâ‚‚.
Engines are not perfect. When the fuel-air mixture is too rich, when combustion temperatures fluctuate, or when the flame doesn’t reach every pocket of fuel in the cylinder, some carbon atoms grab only one oxygen atom instead of two. The result is carbon monoxide, a molecule that still has energy locked inside it because it never finished reacting. COâ‚‚ is the intended destination; CO is what happens when combustion falls short.
How Much CO Actually Comes Out
The ratio of CO to COâ‚‚ in tailpipe emissions is not fixed. It shifts with engine temperature, fuel metering, catalyst condition, and driving style. Measurements of real-world vehicle plumes have found a mean CO-to-COâ‚‚ ratio on the order of about 19 parts per billion of CO for every part per million of COâ‚‚, though this figure was skewed toward cold vehicles, which emit more CO than warmed-up ones.1Atmospheric Environment. The isotopic composition of CO in vehicle exhaust In absolute terms, that means COâ‚‚ dominates the exhaust overwhelmingly, and CO is a trace gas by comparison. But “trace” is misleading when it comes to health risk, because CO is dangerous at concentrations measured in the hundreds of parts per million, while COâ‚‚ becomes a concern only at much higher levels.
The fuel-air equivalence ratio is the single biggest lever on CO production. When an engine runs lean (more air than the fuel needs), CO drops because there is plenty of oxygen to finish the reaction. When it runs rich (excess fuel), CO climbs steeply. Research on four-stroke gasoline engines has shown that improving the air-fuel mixture with intake modifications can cut CO concentrations by more than half at certain engine speeds.2Tibuana. Effect of Using Power Air Screw And Cyclone On Air – Fuel Equivalence Ratio And Monoxide Carbon Gas Emissions In Four Stroke Gasoline Engines
Cold Starts Are the Worst Offender
If you have ever noticed a stronger smell from a car’s tailpipe on a cold morning, that is not your imagination. Cold starts are responsible for a disproportionate share of CO and hydrocarbon emissions. The catalytic converter, which is the main device cleaning up exhaust gases in modern vehicles, needs to reach a working temperature of roughly 350 °C before it begins functioning effectively. Until it gets there, somewhere between half and 80 percent of the total CO and hydrocarbon emissions from a trip pour out of the tailpipe unprocessed.3SAE International. Predicted Cold Start Emission Reductions Resulting from Exhaust Thermal Energy Conservation to Quicken Catalytic Converter Lightoff
This matters practically. Short trips in cold weather, where the engine never fully warms up, produce far more CO per mile than the same engine on a highway cruise. A car that seems “clean” by modern standards can still release concerning amounts of CO during those first few minutes of operation. Hybrid electric vehicles add another wrinkle: their engines shut off and restart repeatedly, and each restart produces a burst of emissions, including elevated particle counts within the first few seconds after the engine fires.4Scientific Reports. Effect of driving characteristics and ambient temperature on the particle emissions during engine restart of spark ignition hybrid electric vehicle
Gasoline Versus Diesel
The balance between COâ‚‚ and CO shifts depending on engine type. Diesel engines generally produce less CO and fewer unburned hydrocarbons than gasoline engines, because diesel combustion occurs with an excess of air (a lean mixture) that allows more complete oxidation of fuel carbon into COâ‚‚.5IOP Publishing. Compression ignition engine – sources of pollution That sounds like good news, and on the CO front it is. But diesel’s more complete combustion also means it converts a larger fraction of its fuel carbon to COâ‚‚, the greenhouse gas. The net COâ‚‚ picture is more nuanced because diesel fuel contains more energy per gallon, so diesel vehicles typically travel farther on the same volume of fuel, producing roughly 24 to 33 percent less COâ‚‚ per mile compared to equivalent gasoline vehicles on a well-to-wheels basis.6PubMed. CO2 emission benefit of diesel (versus gasoline) powered vehicles
Diesel’s trade-off comes elsewhere: higher emissions of nitrogen oxides and soot particles, which create their own serious health and environmental problems. So “diesel is cleaner” is only true along certain axes. It produces less CO, less COâ‚‚ per mile, but more of other pollutants that modern regulations have been specifically targeting.
Why CO Is the Dangerous One
Carbon dioxide at tailpipe concentrations is essentially harmless to a bystander outdoors, though it is the primary driver of climate change at a global scale.7SAE International. The Passenger Car and the Greenhouse Effect Carbon monoxide is a different story entirely. Even at concentrations far below 1 percent, CO can injure or kill. It does this by binding to hemoglobin in your blood far more tightly than oxygen does, forming carboxyhemoglobin. The result is tissue hypoxia: your organs, especially your brain and heart, are starved of oxygen even though you are still breathing.8PubMed. Mechanisms and therapeutic targets of carbon monoxide poisoning: A focus on reactive oxygen species At the same time, CO causes direct cellular damage beyond the hemoglobin effect, compounding the injury.
The insidious part is that CO is colorless and odorless. You cannot smell it, you cannot see it, and the early symptoms of exposure (headache, dizziness, nausea) mimic a dozen common ailments. People in enclosed spaces with running vehicles can lose consciousness without ever realizing what is happening.
The Closed Garage Problem
One of the most persistent real-world CO dangers involves running a car in a closed or poorly ventilated garage. Even a single car started briefly in a small garage can raise CO levels enough to create a health risk.9Results in Engineering. Analysis of carbon monoxide concentration in a single-car garage depending on the method of parking the car – case study The volumes involved are small, CO accumulates quickly, and the danger extends beyond the garage itself when exhaust seeps into living spaces in attached homes.
A common misconception is that modern, catalyst-equipped cars are too clean to be dangerous. Testing has shown otherwise. When a car with a catalytic converter and an oxygen sensor idles in a sealed garage, the declining oxygen levels in the enclosed space eventually fool the engine’s fuel system into running rich, which in turn causes CO output to spike. Both older carbureted vehicles and newer fuel-injected ones with catalytic converters ultimately produced dangerously high CO levels in closed-garage experiments.10SAE International. Automotive Carbon Monoxide Emissions in a Closed Garage The newer car took longer to reach dangerous levels, but it got there. The takeaway is blunt: never idle any internal combustion vehicle in a closed garage, regardless of how modern or clean the car is.
What Catalytic Converters Actually Do
The catalytic converter is the primary reason modern cars produce so much less CO than vehicles from the 1960s and 1970s. Inside the converter, precious metals (platinum, palladium, rhodium) act as catalysts that accelerate chemical reactions without being consumed. CO and unburned hydrocarbons get oxidized into COâ‚‚ and water, while nitrogen oxides get reduced to nitrogen and oxygen. When warmed up and functioning correctly, a three-way catalytic converter can eliminate the vast majority of CO from the exhaust stream.
The key phrase is “when warmed up.” As discussed earlier, the converter is essentially a spectator during cold starts, which is why engineers have spent decades working on ways to get catalysts up to operating temperature faster, including insulated exhaust manifolds, electrically heated catalysts, and close-coupled converter placement near the engine. Despite all that engineering, the cold-start window remains the Achilles heel of automotive emission control.
Converter degradation over the life of a vehicle is another factor. A worn or poisoned catalyst (from contaminated fuel or oil burning, for example) loses efficiency, and CO emissions creep back up. This is one reason many jurisdictions require periodic emissions inspections: to catch vehicles whose converters have degraded enough that CO output has risen above regulatory limits.
How Fuel Choice Shifts the Balance
Blending ethanol into gasoline changes the CO-to-CO₂ equation. Ethanol is an oxygenate, meaning it brings its own oxygen atoms into the combustion chamber. That extra oxygen helps the fuel burn more completely, which tends to push carbon further toward CO₂ rather than stopping at CO. Studies of ethanol-gasoline blends have found that CO and unburned hydrocarbon concentrations in the exhaust decrease as the ethanol fraction rises, while CO₂ concentrations increase.11Energy Conversion and Management. Effect of ethanol–unleaded gasoline blends on engine performance and exhaust emission
Methanol-gasoline blends show a similar pattern. When engines were tested on both ethanol-gasoline and methanol-gasoline blends, CO, CO₂, unburned hydrocarbons, and nitrogen oxide emissions all decreased across the board compared to pure gasoline at a highway cruising speed.12Renewable Energy. Impact of alcohol–gasoline fuel blends on the exhaust emission of an SI engine The fact that both CO and CO₂ dropped in that particular study is worth noting, because it suggests the alcohol blends reduced total carbon throughput (likely by improving thermal efficiency or reducing fuel consumption), not just reshuffling carbon between the two gases.
This is one reason most gasoline sold in the United States contains up to 10 percent ethanol (E10), and higher blends like E15 and E85 are available in some markets. The ethanol content was originally motivated partly by energy independence goals, but the emission benefit is real and measurable.
Small Engines and Non-Road Equipment
If you think modern car emissions are bad during cold starts, small engines on lawn mowers, leaf blowers, chainsaws, and similar equipment occupy a different league altogether. Many of these engines are two-stroke designs or simple four-strokes without catalytic converters, fuel injection, or oxygen sensors. They run rich by default to ensure reliability across varying loads, which means their CO output per unit of fuel burned is often many times higher than a modern car’s.
Emission characterization of small engines has historically received far less research attention than automotive emissions, but the studies that do exist confirm that CO is among the significant pollutants measured in their exhaust.13ACS Publications (Environmental Science & Technology). Measurement of regulated and unregulated exhaust emissions from a lawn mower with and without an oxidizing catalyst: a comparison of two different fuels Adding even a simple oxidizing catalyst to a lawn mower dramatically cuts CO and hydrocarbon output. Regulatory agencies in several countries have been tightening small-engine emission standards in recent years, but the installed base of older, uncontrolled equipment remains enormous. Operating these engines in enclosed or semi-enclosed spaces, like a shed or a poorly ventilated workshop, carries the same CO poisoning risk as a car in a closed garage, sometimes more so because of the higher emission rates.
Electric Vehicles and the Disappearing Tailpipe
Battery electric vehicles produce zero tailpipe emissions of either CO or COâ‚‚, which removes the question entirely for the growing share of vehicles on the road that run on electricity. Plug-in hybrids and conventional hybrids still have internal combustion engines and still produce both gases, but their total output per mile tends to be lower because the electric drive handles a portion of the work, especially at low speeds where combustion engines are least efficient.
The emissions question does not disappear for electric vehicles; it just moves upstream to whatever power plant generated the electricity. In a region powered heavily by coal, the total COâ‚‚ attributable to driving an EV can still be substantial, though CO is no longer a personal exposure concern for the driver or bystanders. In regions with cleaner grids, the lifecycle carbon footprint drops further. The point is that the tailpipe is just one piece of a larger energy chain, and for combustion-powered vehicles, it remains the place where both COâ‚‚ and CO show up in concentrated form.
What Your Nose and Eyes Can (and Cannot) Tell You
Neither COâ‚‚ nor CO is visible. The white cloud you see from a tailpipe on a cold day is water vapor condensing, another normal combustion product. The bluish tint sometimes visible from an older vehicle usually means oil is burning along with the fuel. Black smoke from a diesel typically indicates soot (particulate matter), not CO. None of these visual cues reliably tell you how much CO or COâ‚‚ is present.
Smell is similarly unhelpful for the two gases that prompted the question. COâ‚‚ is odorless at normal concentrations. CO is completely odorless at any concentration. The “exhaust smell” people associate with car fumes comes from other compounds in the mix: unburned hydrocarbons, nitrogen dioxide (which has a sharp, acrid bite), and various volatile organic compounds. A car whose exhaust smells particularly strong might be producing more CO than usual, since rich running conditions tend to produce both CO and unburned hydrocarbons simultaneously, but the smell itself is not the CO. This is precisely why CO detectors in homes and garages are important: your senses cannot do the job.