It all starts in the engine
In order to understand how the exhaust of a vehicle works, there must be a basic understanding of the engine as a whole. The internal combustion engine in its simplest form is a large air pump. It gathers in air, mixes it with fuel, adds a spark and ignites the fuel and air mixture. The keyword here is ‘combustion’. Because the process that makes a vehicle go involves combustion, there are waste products, just as there are waste products associated with any form of combustion. When a fire is lit in a fireplace, the waste products are smoke, soot, and ash. For an internal combustion system, the waste products are gases, carbon particles, and tiny particles suspended in the gases, known collectively as exhaust. The exhaust system filters this waste, and helps it to exit from the vehicle.
While modern exhaust systems are quite sophisticated, that wasn’t always the case. It wasn’t until the Clean Air Act of 1970 that the government had the ability to mandate the amount and type of exhaust that a vehicle produced. The Clean Air Act was modified in 1976, and again in 1990, forcing auto makers to produce vehicles that met a strict set of emissions standards. These laws contributed to the increase in air quality of most major metropolitan areas in the United States, and led to the exhaust system as we know it today.
Parts of the exhaust system
Exhaust valve: The exhaust valve is located in the cylinder head, and it opens after the combustion stroke of the piston.
Piston: The piston forces the gases created from combustion from the combustion chamber into the exhaust manifold.
Exhaust manifold: The exhaust manifold carries the emissions from the piston to the catalytic converter.
Catalytic converter The catalytic converter reduces the amount of toxins in the gases for cleaner emissions.
Exhaust pipe The exhaust pipe carries the emissions from the catalytic converter to the muffler.
Muffler The muffler reduces the noise produced from the combustion and expulsion of exhaust gases
Essentially, the exhaust system works by collecting the waste from the combustion process, then moving it through a series of pipes to different parts of the exhaust system. The exhaust exits the opening created by the movement of the exhaust valve, and is forced into the exhaust manifold. In the manifold, the exhaust from each of the cylinders is collected together, and then forced into the catalytic converter. Within the catalytic converter, the exhaust is partially cleaned. Nitrogen oxides are broken into their respective pieces, nitrogen and oxygen, and oxygen is added to carbon monoxide, creating less toxic, but still dangerous carbon dioxide. Finally, the exhaust pipe carries the cleaner emissions into the muffler, which reduces the associated noises as the exhaust exits into the air.
There’s a long-held belief that diesel exhaust is significantly dirtier than the exhaust from unleaded gasoline. That ugly black smoke belching out of giant exhaust pipes on semi trucks looks, and smells, much worse than what exits the muffler of a car. However, the regulations on diesel exhaust have tightened greatly in the last twenty years, and in most cases, however ugly it may look, diesel exhaust is just as clean as what exits from a gas-fueled vehicle. Diesel particulate filters remove 95% of a diesel-fueled vehicle’s smoke (source: http://phys.org/news/2011-06-myths-diesel.html) meaning that what you see is more soot than anything else. In fact, exhaust from diesel engines contains less carbon dioxide than exhaust from gas engines. Because of the tighter controls on diesel emissions, as well as the increased mileage considerations, diesel engines are being used more often in smaller vehicles, including models by Audi, BMW, and Jeep.
Most common symptoms and repairs
Exhaust system repairs are common. When there are so many moving parts in one system that works constantly, there are bound to be common repairs.
Cracked exhaust manifold: A vehicle can have a cracked exhaust manifold which will sound like a loud ticking sound from near the areas of the engine, that will sound like a giant clock.
Faulty donut gasket: Will also be a loud ticking sound, but can usually be heard coming from underneath the car when an occupant is sitting in the car with the door open.
Clogged catalytic converter: Will present itself in a dramatic loss of power and strong odor of something burning.
Rusted exhaust pipe or muffler: The sound of the exhaust leaving the muffler will get noticeably louder.
Faulty O2 sensor: The check engine light on the instrument panel will illuminate
Upgrading a vehicle’s exhaust system
There are multiple upgrades can be done to the exhaust system, for the purposes of increasing performance, augmenting sound, and improving efficiency. Efficiency is important for a smooth running vehicle and these upgrades can be done by certified mechanics who will order replacement exhaust system parts that match the originals on the vehicle. When talking performance, there are performance exhaust systems which can give the vehicle a boost in horsepower, there are even some that can help with fuel economy. For these repairs, an entirely new exhaust system would need to be installed. In regards to sound, a vehicle can go from the stock sound to a sound that can be best described as throaty, going so far as to give the vehicle a sound comparable to a roar. Do not forget that when the exhaust gets upgraded, the intake needs to be upgraded as well.
This article originally appeared on YourMechanic.com as How Exhaust Systems Work and was authored by Keisha Page.