Why Modern Diesel Engines Need an After-Treatment System
If you’ve worked on diesel engines for any length of time, you’ll know one thing for certain, diesel engines are incredibly efficient.
For decades, that efficiency made them the engine of choice for trucks, buses, agricultural machinery and industrial equipment. They produced excellent torque, impressive fuel economy and, when properly maintained, could achieve extraordinary service life.
So why did manufacturers suddenly begin fitting diesel particulate filters, SCR catalysts, AdBlue systems and a growing collection of sensors to what had previously been a relatively simple engine?
The answer begins inside the combustion chamber.
Combustion Is Never Perfect
Every diesel engine has one job, convert the chemical energy stored in diesel fuel into useful mechanical work.
In an ideal world, the products of combustion would simply be:
- Carbon dioxide (CO₂)
- Water vapour (H₂O)
- Nitrogen (N₂)
Unfortunately, combustion inside a diesel engine is far from ideal.
The combustion process occurs under extremely high temperatures and pressures, with fuel being injected directly into compressed air in a matter of milliseconds. Although modern injection systems have become remarkably precise, perfect combustion simply doesn’t exist.
Instead, combustion produces several unwanted by-products known as emissions.
The Four Main Pollutants
Modern after-treatment systems are designed to reduce four primary pollutants.
Carbon Monoxide (CO)
Carbon monoxide is produced when fuel does not burn completely due to insufficient oxygen or incomplete combustion.
It is colourless, odourless and toxic.
Fortunately, it is relatively easy to convert into carbon dioxide using an oxidation catalyst.
Hydrocarbons (HC)
Hydrocarbons are simply unburned fuel molecules that leave the engine before combustion is complete.
Like carbon monoxide, hydrocarbons can be oxidised into carbon dioxide and water.
Particulate Matter (PM)
Particulate matter, more commonly known as soot, consists of microscopic carbon particles produced during combustion.
These particles are responsible for the black smoke that older diesel engines were famous for under heavy acceleration.
Unlike carbon monoxide and hydrocarbons, soot cannot simply be converted inside the engine. Instead, it must be physically captured and periodically burned away.
This is the job of the Diesel Particulate Filter (DPF).
Nitrogen Oxides (NOx)
Nitrogen oxides are perhaps the most interesting of all diesel emissions.
Air entering the engine is made up of approximately 78% nitrogen and 21% oxygen.
Under normal conditions these gases coexist happily without reacting.
However, inside the combustion chamber, temperatures can exceed 1800°C.
At these temperatures, nitrogen and oxygen begin reacting chemically to form nitric oxide (NO) and nitrogen dioxide (NO₂), collectively referred to as NOx.
Ironically, the conditions that produce the highest engine efficiency also produce the highest levels of NOx.
This presents engineers with a difficult challenge.
The Engineering Compromise
Imagine you could design an engine with only one objective…. Maximum fuel economy…. The solution would be relatively straightforward.
Now imagine your objectives become:
- Maximum fuel economy.
- Maximum power.
- Lowest possible emissions.
- Lowest fuel consumption.
- Long engine life.
- Excellent drivability.
- Reliable cold starting.
- Low noise.
Suddenly, every engineering decision becomes a compromise:
- Increasing combustion temperature generally improves efficiency, but increases NOx
- Reducing combustion temperature lowers NOx, but may increase soot
- Reducing soot often requires additional oxygen
- Reducing NOx often requires less oxygen
These objectives directly oppose one another.
Modern engine management is therefore a constant balancing act.
The Role of After-Treatment
Instead of trying to eliminate every pollutant inside the engine, manufacturers adopted a different approach…. Allow the engine to operate efficiently, then clean the exhaust before it reaches the atmosphere.
This is known as the after-treatment system.
Rather than being a single component, modern after-treatment is a series of chemical and mechanical processes working together.
- Each component has a specific purpose
- Each sensor provides information to the Engine Control Module (ECM)
- Each actuator is carefully controlled to achieve the desired result.
Understanding these components individually is important, and understanding how they work together is what separates a parts replacer from a diagnostician.
Looking Ahead
Before we can diagnose emissions faults, we first need to understand:
- How the engine itself breathes
- How does the ECM know how much air has entered the engine
- Why do some engines use a Mass Air Flow (MAF) sensor while others rely on pressure and temperature sensors?
- What exactly are MAP sensors measuring?
- And why can two engines use completely different sensor strategies while achieving the same result?
These are the questions we’ll answer in Part 2, where we’ll begin with the one thing every diesel engine depends on:
Air.