RECDs for Turbocharged vs Naturally Aspirated DG Engines

Ask a generator technician what sits between the exhaust manifold and the silencer, and on most sets above a modest size the answer will be a turbocharger. On many small sets, and on plenty of older ones, the answer will be nothing at all: the exhaust leaves the manifold and heads straight for the pipe.

That one component changes how an engine breathes, how hot its exhaust runs at different loads, how it reacts when a large motor starts, and how much extra exhaust resistance it can tolerate. Every one of those traits matters to a retrofit emission control device, because the RECD sits directly downstream of whatever the engine sends out.

This guide compares the two engine types from the point of view of the device that has to live with them. It starts with how to identify which type you have, then works through five behaviours where the two diverge, and finishes with a retrofit checklist for each.

Two Ways to Fill a Cylinder

A diesel engine burns fuel in air, and the amount of power it can make is limited by how much air it can get into each cylinder.

A naturally aspirated engine relies on the piston moving down to draw air in at roughly atmospheric pressure. Nothing forces extra air in. The engine’s output is limited by its displacement, its speed and how freely its intake and exhaust passages flow.

A turbocharged engine uses a turbine, spun by exhaust gas, to drive a compressor that pushes air into the cylinders above atmospheric pressure. More air means more fuel can be burned cleanly, so the same size of engine can make much more power. Many generator engines add an aftercooler (also called an intercooler or charge air cooler) to cool the compressed air before it enters the cylinders, since cooler air is denser. Aftercoolers may be air-to-air, using a separate core in front of the radiator, or jacket-water and separate-circuit types that use coolant. A good general description of the device is in the Wikipedia entry on turbochargers.

The key point for emission control is that a turbocharger is not just a performance part. It sits in the exhaust stream, takes energy out of it, and links what happens in the exhaust pipe back to how much air the engine receives.

How to Tell Which Type Your Set Has

You do not need to open anything. Look for these signs:

Sign	Naturally aspirated	Turbocharged
Turbocharger housing	None; exhaust manifold connects straight to the exhaust pipe	Snail-shaped housing between manifold and pipe, often wrapped in a heat shield
Air intake path	Air filter connects directly to the intake manifold	Air filter connects to the compressor inlet, then a pipe to the intake manifold, sometimes through a cooler
Charge air cooler	None	Often a second core in front of or beside the radiator, or a cooler mounted on the engine
Engine model code	Varies by maker; check documentation	Often includes letters such as T, TA, TC, TCA or TAA
Typical sizes in Indian DG sets	Mostly smaller and older sets	Most mid-size and large sets

The model code is a good clue but not a rule, since each maker uses its own convention. The engine’s data sheet will state the aspiration directly. If you are preparing photos for an RECD enquiry, include one that shows the turbocharger area, or the lack of one; our guide on DG nameplate photos for an RECD enquiry lists the other shots vendors need.

Behaviour 1: Where the Exhaust Heat Goes

A particulate filter cleans itself during normal running when trapped soot burns off, a process called passive regeneration. It relies on exhaust that is hot enough, assisted by a catalyst coating that helps soot oxidise at lower temperatures than it would on its own. Our explainer on what a diesel particulate filter is and how it helps describes the mechanism.

In a turbocharged engine, the turbine extracts energy from the exhaust before it reaches the RECD. Gas leaving the turbine is noticeably cooler than gas entering it. The RECD sees the post-turbine temperature, not the manifold temperature.

There is a second effect. Turbocharged engines usually run with more excess air than naturally aspirated ones, especially at part load. More air for the same fuel means a cooler mixture after combustion.

A naturally aspirated engine has no turbine to take heat out and less spare air. At high load, its exhaust tends to run hot. That is helpful for regeneration but has a cost: with no forced air in reserve, an NA engine pushed hard approaches the point where it cannot burn all its fuel cleanly, and smoke rises.

What it means for the RECD:

• On a turbocharged engine, check the exhaust temperature at the turbine outlet at your typical load, because that is what the device will see. Light-load running on a turbocharged engine can give quite cool exhaust.
• On an NA engine, temperatures at moderate to high load are often favourable for regeneration, but high soot output near full load can offset that advantage.

Behaviour 2: Sensitivity to Exhaust Backpressure

Every engine has a maximum allowable exhaust backpressure set by its maker, and the RECD adds to the resistance of the exhaust system. How the two engine types respond to extra resistance differs in an important way.

Naturally aspirated engine. Higher backpressure makes it harder for the piston to push exhaust out. More burned gas stays in the cylinder, leaving less room for fresh air on the next stroke. The engine loses some power and efficiency and may smoke more. The effect is real but fairly direct.

Turbocharged engine. Higher backpressure at the turbine outlet reduces the pressure drop across the turbine. The turbine slows, the compressor delivers less boost and the engine gets less air. Less air with the same fuel demand means a richer mixture: more smoke, higher exhaust temperature and, in time, more stress on valves, turbocharger and pistons. In other words, on a turbocharged engine, extra backpressure feeds back into the air supply.

This does not mean turbocharged engines cannot take an RECD. Millions of turbocharged engines worldwide run with particulate filters. It means the backpressure calculation matters a great deal, and the device must be sized so that the total system stays inside the limit even when the filter carries a normal soot load.

Our article on how exhaust backpressure affects a DG set explains measurement and limits. For turbocharged engines, ask the vendor for the predicted backpressure at both clean and loaded filter conditions, and plan to measure it during commissioning.

What it means for the RECD:

• For a turbocharged engine, treat the OEM backpressure limit as a hard boundary and insist on a calculation for your full exhaust route.
• For an NA engine, the same calculation is needed, but the consequences of a small overshoot tend to show as gradual power loss rather than a change in air supply.

Behaviour 3: Smoke When the Load Jumps

Generator loads do not always rise gently. A large motor starting, a lift beginning its run, a chiller compressor kicking in or a full building transferring to DG after a grid failure can all hit the engine with a sudden step.

Turbocharged engine. When the load jumps, the governor immediately increases fuel. The turbocharger, however, needs a moment to spin up, because it depends on exhaust energy that has not yet increased. For a short interval, the engine has extra fuel but not yet the extra air. The result is a brief puff of black smoke, commonly called turbo lag smoke. Modern engines with electronic fuel control limit this, but many older mechanical engines show it clearly. Generator makers describe load-step capability under ISO 8528-5, and Kohler’s white paper on ISO 8528-5 and generator transient performance shows why turbocharged engines with high output per litre often accept smaller first load steps.

Naturally aspirated engine. There is no lag in the same sense, because air supply does not depend on a turbine. But there is also no reserve. If the step pushes the engine near its full rating, it may smoke because it simply cannot get more air.

For the RECD, both cases mean bursts of soot. The filter traps it, which is exactly its job. The question is how often these bursts happen and how much soot they add between regeneration periods. A set at a factory with frequent motor starts may load its filter faster than a set carrying steady lighting and office loads.

Our guide to why a generator emits black smoke covers causes beyond load steps. If smoke on load steps is heavy, check the turbocharger, air filter and fuel injection before fitting an RECD.

What it means for the RECD:

• Tell the vendor about large motors, lifts and other step loads, particularly on turbocharged engines with mechanical fuel systems.
• Consider soft starters or variable frequency drives on large motors where practical. They reduce the step seen by the engine, which helps the engine as well as the filter.

Behaviour 4: Light Load and Oil

Light-load running is hard on diesel engines in general, and the effects are covered in our article on understanding generator load factor and duty cycle. Turbocharged engines have an extra concern.

A turbocharger’s shaft spins in bearings fed with engine oil. Seals keep that oil out of the compressor and turbine housings. These seals work best when there is positive boost pressure on the compressor side and normal gas pressure on the turbine side. At very light load and long idle periods, boost is minimal. Under those conditions, some turbochargers can pass small amounts of oil into the intake or exhaust stream, and the problem gets worse as seals and bearings wear.

Oil in the exhaust matters to an RECD for two reasons:

1. Wet soot. Unburned oil and fuel combine with soot into a sticky deposit, sometimes called wet stacking, which can coat the inlet face of the device.
2. Ash. Lubricating oil contains additives that leave incombustible ash when burned. Ash does not regenerate away. It gradually accumulates in the filter and can only be removed by cleaning.

Naturally aspirated engines also burn oil when piston rings and valve guides wear, and they also suffer at light load. But they do not have a turbocharger seal as an additional oil path.

What it means for the RECD:

• For a turbocharged set that runs light for long periods, check for oil in the intake piping and at the turbine outlet before fitting.
• Avoid long no-load runs. Replace them with loaded exercise runs.
• Track oil consumption. A rise often appears before visible smoke.

Behaviour 5: Exhaust Volume per kW

At full load, turbocharged engines usually run with more air per unit of fuel than naturally aspirated engines. More air means more exhaust mass for the same power output, even though that exhaust is cooler.

Exhaust volume, which is what the RECD’s filter area and catalyst volume must handle, depends on both mass and temperature. Hot gas takes up more space than cool gas. So a cooler but larger mass of turbocharged exhaust and a hotter but smaller mass of NA exhaust can end up closer in volume than you might expect.

The practical answer is not to guess. Use the engine maker’s published exhaust flow and temperature at your typical loads. Our companion guide on using OEM exhaust flow data when specifying an RECD explains how to read and convert those figures.

Side-by-Side Summary

Factor	Naturally aspirated	Turbocharged (with or without aftercooler)	RECD design response
Exhaust temperature at the device	Higher at moderate and high load	Lower, because the turbine takes out heat and excess air dilutes it	Check catalyst light-off and regeneration at real load
Response to extra backpressure	Gradual power loss, more residual gas	Reduced boost, richer mixture, more smoke and heat	Tight backpressure calculation, especially for turbo
Load-step smoke	Smoke if step approaches full load	Turbo lag puffs, more on older mechanical engines	Allow for soot bursts; reduce step loads where possible
Light-load issues	Wet stacking, cool exhaust	Same, plus possible oil past turbo seals	Loaded exercise; check for oil before fitment
Exhaust mass per kW	Lower	Higher	Size from OEM flow data, not assumptions
Typical presence in India	Smaller and older sets	Most mid-size and large sets	Match device range to engine type

Retrofit Checklist: Turbocharged Engines

Before fitting an RECD to a turbocharged set, work through these checks with your service provider and the vendor.

Turbocharger health

• Spin check with the engine off and cool: the shaft should turn freely without rubbing or excessive play.
• Look for oil in the compressor outlet pipe and at the turbine outlet flange.
• Listen for whistling or rubbing under load.
• Check that the turbocharger is the correct model for the engine. A replacement turbocharger from a different source can change airflow and backpressure behaviour.

Charge air system

• Inspect charge air pipes and hoses for leaks, cracks and loose clamps. A boost leak starves the engine of air and raises smoke.
• Clean the aftercooler core. A blocked air-to-air core reduces cooling and air density.
• For water-cooled aftercoolers, check the coolant circuit.

Air intake

• Check air filter restriction. A clogged filter starves the compressor. Our generator air filter maintenance guide explains how.

Exhaust baseline

• Measure exhaust backpressure at the turbine outlet at full or highest practical load, before any change. This baseline shows how much of the budget the existing system already uses.
• Note exhaust temperature at the turbine outlet at typical load.

Load profile

• List large motors and other step loads.
• Collect load data showing typical running level and run lengths.

After fitment

• Measure backpressure again at load and compare with the OEM limit.
• Watch for any increase in smoke on load steps, which could point to an air supply issue.
• Review the device’s backpressure trend over the first few weeks.

Retrofit Checklist: Naturally Aspirated Engines

NA engines are simpler, but they have their own priorities.

Engine condition

• Check for blue smoke, which suggests oil burning. Our guide to why a generator emits blue smoke explains causes.
• Check injector condition and timing. NA engines with poor injection smoke readily.
• Check compression if the engine has high hours.

Load headroom

• Confirm the set is not regularly run close to or above its rating. NA engines smoke heavily when overloaded, and that soot goes straight to the filter.
• If peak loads are near the limit, consider load management or a larger set.

Exhaust route

• Small NA sets often have compact canopies with short pipes. Make sure there is space for the device and safe access to it.
• Check the backpressure limit; small engines can have limits that leave little margin for long pipe runs.

Regulatory band

• Many NA engines sit at the small end of the generator range. In NCR, CAQM Direction No. 76 linked compliance routes to engine power bands, with dual fuel as the route stated for 19 to 125 kW sets. Check how the current rules apply to your set before choosing between an RECD and a dual fuel kit; our RECD vs dual fuel kit comparison helps.

Special Cases Worth Flagging

Twin-turbo V engines

Large V engines often have one turbocharger per bank and two exhaust outlets. Each bank must stay within its backpressure limit. If the banks are merged into one device, the merger must not create unequal pressure between banks. Two devices, one per bank, are often the simpler choice.

Wastegated turbochargers

Some turbochargers have a wastegate, a valve that lets exhaust bypass the turbine at high boost. Extra backpressure downstream can affect how the wastegate behaves. Mention it to the vendor if your engine has one.

Engines converted or repowered

Some older sets have been repowered with a different engine, or have had a turbocharger added or removed during repairs. The original set documents then no longer describe the engine. Specify the RECD from the engine actually fitted. Our guide on specifying an RECD when original DG set documents are missing helps when paperwork does not match.

Imported engines

Imported sets may use turbocharger and aftercooler arrangements less common in India, with data sheets in different units. The article on whether an RECD can be retrofitted to an imported diesel generator covers the extra checks.

Engines with existing after-treatment

Newer CPCB IV+ engines may already have a particulate filter or other after-treatment. Adding an RECD to such an engine is a different question, discussed in whether an RECD can be added to a generator with an existing DPF or SCR.

Myths About Turbochargers and RECDs

“A turbocharged engine is cleaner, so it needs a smaller RECD.”

Turbocharging allows cleaner combustion at a given power, and modern turbocharged engines often produce less soot per kWh than older NA designs. But the device must still handle the full exhaust volume, which is often larger. Soot rate and exhaust volume are separate sizing inputs.

“The RECD will damage the turbocharger.”

A correctly sized RECD that keeps total backpressure within the OEM limit should not harm the turbocharger. Problems arise when devices are undersized, exhaust routes are ignored or filters are left to overload. Our article on whether installing an RECD affects the generator’s OEM warranty discusses how to protect yourself.

“NA engines run hot, so their filters never clog.”

Higher exhaust temperature at load helps, but NA engines also tend to produce more soot, particularly when worn or near full load. Light running hurts them just as it hurts turbocharged engines.

“You can tell the aspiration from the kVA.”

Not reliably. Different makers use different engines for the same set rating, and some ranges have overlapping NA and turbocharged options. Check the engine.

Frequently Asked Questions

Can an RECD be fitted to a turbocharged DG engine?

Yes. Turbocharged engines are the most common type in mid-size and large DG sets, and RECDs are routinely fitted to them. The device must be sized so that total exhaust backpressure stays within the engine maker’s limit, because extra resistance on a turbocharged engine reduces boost and air supply.

Is an RECD suitable for a naturally aspirated generator?

Yes, if the engine is in reasonable condition and not regularly overloaded. Check whether your set’s power band calls for an RECD or another route under current rules.

Why does my turbocharged generator puff black smoke when a motor starts?

The turbocharger needs a moment to spin up after a sudden load increase, so for a short time the engine has extra fuel without extra air. Older mechanical engines show this more. An RECD traps that soot, but heavy puffs suggest checking the turbo, air filter and injection.

Does a turbocharger make the exhaust cooler for the RECD?

Usually yes. The turbine takes energy out of the exhaust, and turbocharged engines run with more excess air. That lowers the temperature at the device, which matters for passive regeneration, particularly at light load.

Can a worn turbocharger affect the RECD?

Yes. A worn turbocharger can pass oil into the exhaust, adding wet deposits and ash to the filter, and can reduce airflow, raising soot. Check and repair it before fitment.

Should I measure backpressure before and after fitting an RECD?

Yes, especially on turbocharged engines. A baseline before fitment shows how much margin the existing system leaves, and a reading after fitment confirms the device keeps the total within the limit.

Match the Device to the Way the Engine Breathes

Whether your engine pulls in air on its own or has it forced in by a turbocharger changes the temperature, volume and behaviour of the exhaust that reaches an RECD. Turbocharged engines call for close attention to backpressure, turbo health and step-load smoke. Naturally aspirated engines call for attention to overload, wear and the right compliance route. In both cases, real load data and the engine maker’s figures lead to the right device.

Aceget supplies CPCB type-approved RECDs for DG sets from 25 kVA to 2500 kVA, covering both naturally aspirated and turbocharged engines. Smaller sets can start with the 62.5 kVA DG set RECD, while large turbocharged sets can refer to the 1010 kVA DG set RECD. To discuss your engine, get in touch with Aceget or explore the full retrofit emission control device range.