A detailed look at what's coming out of your tail pipe

A detailed look at what's coming out of your tail pipe  
Filed under:
on 06/20/2008

You should have come across terms like CO2/km, Euro V or CAFE Norms. These terms are featured in every other auto news blog or website on an everyday basis and leaves you wondering what they exactly are. In this Automotoportal featured, we talk about types of emissions emerging from your vehicle's tailpipe and everything else associated with it. The accompanying diagram will familiarize you on all the terms mentioned throughout this article. So sit back and enjoy this simplified article!

Sources of emissions:

1) Engine Crankcase
2) Gasoline tank
3) Fuel reservoir
4) Exhaust

Hydrocarbons (HC)

Fuel that you fill the tank with consists of hydrogren and carbon. Better known as unburnt Hydrocarbons, the main source is the fuel itself. During the cycle of operation, the fuel entering the cylinder escapes without being combusted during the power stroke.

HC's also originate from the fuel tank and the crankcase. The former is due to higher surrounding temperature (evaporation) and the later is due to the escaping of fuel into the crankcase through the gap between the cylinder and the piston (Refer diagram).

Click to enlarge

The following will give you an idea of the sources responsible for HC emission.

60%- Exhaust
20%- Crankcase
20%- Fuel tank vents and fuel system

Complete combustion of gasoline yields only water and carbon dioxide alongside energy. But it is not possible to create a setup for complete and perfect combustion to happen. When the vaporized and compressed charge in the cylinder is ignited during the power stroke by the spark plug, the flame passes through the mixture but misses out certain region close to the cylinder walls. This region comprises of some amount of unburnt fuel which is pushed into the exhaust during the exhaust stroke.

This problem is magnified when the engine misfires. HC emissions are a reason for photochemical smog which poses a threat to visibility, especially in tunnels and enclosed areas leading to serious accidents.

Fuel tank losses occur due to the dissipation of fuel vapor into the surrounding atmosphere when refilling or vaporization of fuel in tank which escapes through the breather vent. Evaporation of fuel from the tank is determined by a number of factors:

1) Surrounding and fuel tank temperature- When the car is parked in shade, the losses are minimized.
2) Mode of vehicle operation
3) Amount of fuel in the tank- Lesser the fuel, more the losses due to increase in vapor pressure.

Crankcase Blowby

Look closely at the diagram. There is a small clearance between the piston and the cylinder termed "clearance volume." This is a passage and the fresh air/fuel mixture entering the cylinder can pass through this and enter the crankcase (lower portion of the cylinder, above the oil sump) This is termed as "blow by" and it accounts to 20% of HC emissions. If the piston rings wear out, which is the case when the engine has operated for many thousands of kilometers, the phenomenon is raised to around 30%. This means there is wastage of fuel and power let alone higher emissions.


Probably the worst of the lot, Nox formation is simply due to
a) High temperature
b) Availability of excess air

Nitrogen makes up more than 70% of the air we breathe. It is an inert gas (non-reactive and stable) at room temperature. But when the temperature in the combustion chamber reaches 1100 degree centigrade, things are entirely different. Nitrogen and Oxygen combine to for NOx . NOx is harmful to humans as it causes respiratory problems and asthma. It also leads to the formation of acid rain which is deadly to living things.

So if lowering the combustion chamber temperature would curb NOx, why won't automakers design it like that? The reason is simple and obvious. If the temperature of the chamber is brought down, HC emissions would rise as the fuel won't burn entirely and the overall efficiency of the engine would fall if temperature scales down (Thermodynamic principle).

When the ratio of NOx to HC reaches a particular level, the two mix in the presence of sunlight to form photochemical smog (explained earlier)

CO (Carbon Monoxide)

CO is a poisonous gas. The 2 characteristics that make it toxic and deadly are- it is colorless so you can't see it and it is odourless, meaning you can't smell it or sense it if it's around. If a person is exposed to CO for a period of time, death is definite. Headache and blurry vision are the first symptoms when exposed to CO.


When there is sufficient oxygen, the fuel (HC) is completely oxidized so that all carbon atoms combine with oxygen to form carbon dioxide and all hydrogen atoms combine with oxygen to form water. When the air-fuel mixture entering the cylinder is rich, ie- composed more of gasoline, there isn't enough oxygen in the combustion chamber for the combustion to yield C02. Instead CO is formed. CO is not only an emission but also represents wasted energy. A rich mixture is required whenever you need acceleration or when you start the engine after a whole night's rest.

CO is formed when:

a) Insufficient amount of air in air/fuel mixture
b) Insufficient time in the cycle for completion of combustion

CO emissions are high during idling and attain their highest value when decelerating from high speeds as oxygen supply to the engine is completely cut off.

Smoke from Diesel engines

One of the common sights on roads today is the smoke emission from diesel engined vehicle like heavy duty trucks and busses. There are 2 types of smoke from diesel engines-

1) Black smoke - This is mainly due to the carbon particles present in the exhaust gas. During sudden acceleration, the fuel entering the cylinder is more in quantity but the availability of air for that fuel to burn entirely is not present. The fuel has to mix and vaporize with the air which is not possible when there is a sudden increase in the amount of fuel metered into the cylinder.

2) Blue-White smoke - Smoke of this color occurs during cold start. When starting the engine early in the morning or after a entire night's time period, the surrounding temperature in the cylinder is low and complete combustion of the charge is not possible.

Also when the lubricating oil enters the combustion chamber, it burns along with the air/fuel mixture generating a bluish white smoke. This is even more so when the piston ring wear is more and is a direct indication that maintenance or immediate care is required.

How you have a direct connection to some of the above emissions:

As a car owner, you must take good care of the car. Regular maintenance has a direct impact on what comes out of the tailpipe and how much you end up spending at the pump. Change your spark plug at prescribed times. A faulty spark plug results in misfires where the incoming fuel goes out of the chamber without getting combusted. The result in increased HC emissions. Spark advance and retard mechanisms have a direct control over the formation of NOx and so check with your mechanic whether the ignition system is working satisfactorily. A dirty air cleaner means you are choking the engine from getting its faire share of air, which will promote HC emissions. Oil changes are vital and make sure you change your engine oil as recommended by your car's manufacturer. If the oil gets too old, it will lose its lubricating property tending to accelerate ring wear leading to detrimental effects. Be nice to your car. After all, it's there to make your life simpler!

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