How to Use OEM Exhaust Flow Data When Specifying an RECD

Somewhere in the paperwork for your generator, possibly in a ring binder in the DG room or a PDF on the maker’s website, there is a page that matters more to your RECD than the nameplate does. It is the engine’s technical data sheet, and halfway down it, usually under a heading such as “Exhaust System”, there are three or four lines: exhaust gas flow, exhaust gas temperature, maximum allowable backpressure and perhaps the outlet size.

Those few lines are the closest thing to a direct measurement of what your RECD will face. They come from the engine maker’s own test cells. Used properly, they let a vendor size the device, check backpressure and predict regeneration with confidence. Used carelessly, they cause some of the most common mistakes in RECD specification: devices sized on the wrong units, the wrong rating column or the wrong temperature.

This is a field guide to those lines. It shows where to find the data, explains each field, flags the unit traps, works through a conversion, and ends with a table you can copy into a request for quotation.

Where OEM Exhaust Data Lives

Engine and generator makers publish exhaust data in several places. Check them in roughly this order.

1. Generator set specification sheet. The genset maker’s brochure or spec sheet often includes a short engine section with exhaust flow and temperature at rated output.
2. Engine performance data sheet. The engine maker’s own data sheet for your specific engine model and rating is usually the most complete source, with values at several load points.
3. Installation manual or application guide. These often give the maximum exhaust backpressure, recommended pipe sizes and exhaust outlet details.
4. Dealer or service partner technical support. If you give the engine serial number, an authorised dealer can usually retrieve the data sheet that matches your build.
5. Original project documents. Consultants’ specifications and handover files for the building may contain the data sheet that was approved at purchase.

When you send photos for an RECD enquiry, include the engine serial plate so the data can be matched. Our guide on DG nameplate photos for an RECD enquiry shows which plates to capture.

Anatomy of the Exhaust Section

Data sheets vary by maker, but most include the fields below. The middle column explains what each means; the right column explains why the RECD cares.

Field on the data sheet	What it means	Why it matters for the RECD
Exhaust gas flow	Volume or mass of exhaust leaving the engine per unit time, at a stated load	Sets filter and catalyst frontal area and volume; drives pressure drop
Exhaust gas temperature	Temperature of exhaust at a stated point, usually the turbine outlet or exhaust outlet, at a stated load	Governs passive regeneration and catalyst activity; converts volume between conditions
Maximum allowable exhaust backpressure	Highest resistance the engine maker allows in the exhaust system, measured at a stated point	Sets the budget for the whole exhaust system including the RECD
Exhaust outlet size	Diameter or flange size of the engine’s exhaust connection	Determines connection design and whether transitions are needed
Number of outlets	One for most inline engines, often two for V engines	May mean two devices or a merger
Combustion air flow	Air drawn into the engine	Cross-check for exhaust flow, since exhaust mass is roughly air mass plus fuel mass
Fuel consumption	Fuel burned at each load point	Completes the mass balance
Load points	Values at 25%, 50%, 75%, 100% and sometimes 110% of a rating	Shows how flow and temperature change across the operating range
Rating type	Standby, prime or continuous	Tells you which power level the values refer to
Reference conditions	Ambient temperature, pressure and humidity for the stated values	Lets you adjust for hot or high sites

Not every data sheet has all of these. Older engines in particular may give only full-load figures. That is still useful, as long as you know exactly what the figures represent.

The Five Unit Traps

Most errors in using exhaust data come from five sources. Check each one every time.

Trap 1: Actual, normal or standard cubic metres

Exhaust volume changes with temperature and pressure. A data sheet may quote flow as:

• Actual volume (m³/min, m³/h, cfm or acfm) at the stated exhaust temperature and approximately atmospheric pressure
• Normal volume (Nm³/h), corrected to 0°C and 1.013 bar
• Standard volume (scfm or Sm³/h), corrected to a reference such as 15°C or 20°C, depending on the maker’s convention

The difference is large. Exhaust at 500°C occupies nearly three times the volume it would at 0°C. If one vendor sizes a device on actual flow and another on normal flow, their designs will look very different for the same engine. Always confirm which basis the data sheet uses, and convert all figures to one basis before comparing.

Trap 2: Mass flow or volume flow

Some makers give exhaust flow in kg/h or lb/min. Mass flow does not change with temperature, which makes it convenient for engineering, but you need temperature and pressure to turn it into volume. If one data sheet gives mass and another gives volume, do not compare them directly.

Trap 3: Imperial and metric volume units

The common conversions:

From	To	Multiply by
cfm (ft³/min)	m³/h	1.699
cfm	m³/min	0.02832
m³/min	m³/h	60
lb/min	kg/h	27.22

Trap 4: The wrong rating column

Many data sheets show separate columns for standby and prime ratings. The standby column describes higher output for limited emergency use; the prime column describes output for longer running. Cummins explains the categories in its note on understanding ISO 8528 generator set ratings. For backpressure checks, design for the highest flow the engine will actually produce in service. For regeneration, look at the flow and temperature at the load where the set spends most of its hours. Make sure the column you use matches how your set is rated and operated.

Trap 5: Where the temperature was measured

On turbocharged engines, exhaust temperature before the turbine is much higher than after it. The RECD sits after the turbine, so it is the turbine outlet temperature that matters. Most generator data sheets quote turbine outlet or exhaust outlet temperature, but some engineering documents give manifold temperature. If the figure looks unusually high, check.

And one more: backpressure units

Maximum backpressure appears in many units. Convert carefully:

Unit	Equivalent in kPa
1 inch of water (in H₂O)	0.249 kPa
1 mm of water (mm H₂O, or mm WC)	0.00981 kPa
1 mbar	0.1 kPa
1 inch of mercury (in Hg)	3.386 kPa
1 psi	6.895 kPa

So a limit of 40 in H₂O is about 10 kPa, a limit of 27 in H₂O is about 6.7 kPa, and a limit of 1,000 mm WC is about 9.8 kPa.

A Worked Conversion

Suppose a data sheet for a turbocharged engine driving a 200 kVA set shows, at 100% prime rating:

• Exhaust gas flow: 1,180 cfm
• Exhaust gas temperature: 510°C
• Maximum allowable backpressure: 40 in H₂O

Convert flow to metric actual volume

1,180 × 1.699 ≈ 2,005 m³/h actual, or about 33.4 m³/min

Convert to normal volume

Normal volume = actual volume × (273 ÷ exhaust temperature in K) × (actual pressure ÷ 1.013 bar)

Exhaust temperature in K = 510 + 273 = 783 K. Taking pressure as close to atmospheric:

2,005 × (273 ÷ 783) ≈ 2,005 × 0.349 ≈ 699 Nm³/h

Estimate mass flow

Diesel exhaust at normal conditions has a density close to that of air, about 1.29 kg/Nm³:

699 × 1.29 ≈ 902 kg/h

Convert backpressure

40 × 0.249 ≈ 10 kPa

Now you have the engine’s full-load exhaust described in three consistent ways, plus the backpressure budget in kPa. A vendor quoting in any of these units can be checked against them.

Sense-check with the fuel

Exhaust mass is roughly air mass plus fuel mass. If the same data sheet lists fuel consumption of about 44 litres per hour at full prime load (roughly 37 kg/h), the implied air flow is about 865 kg/h, giving an overall air-fuel ratio of around 23:1. That sits in a plausible range for a turbocharged generator engine at full load. If the arithmetic produced a ratio of 8:1 or 60:1, you would know something had gone wrong with the units.

Reading Across the Load Points

A good data sheet gives values at several loads. Here is an illustrative set for the same engine.

Load (% of prime)	Exhaust flow (m³/h actual)	Exhaust temperature (°C)	Normal flow (Nm³/h)
25%	About 900	About 300	About 430
50%	About 1,300	About 390	About 535
75%	About 1,650	About 455	About 620
100%	About 2,005	About 510	About 699

Three uses of this table:

1. Backpressure sizing uses the top row that the engine will actually reach. Pressure drop across a filter rises with flow, so the highest flow in service sets the worst case. Add the soot loading allowance on top.
2. Regeneration planning uses the row where the set spends most of its time. If the set mostly runs at 30 to 40% load, the temperature around that point matters far more than the full-load figure. Our article on understanding generator load factor and duty cycle explains why. The companion guide on how DG load logs help assess RECD suitability shows how to find that point from real data.
3. Interpolation fills the gaps. If your typical load is 40%, estimate between the 25% and 50% rows. Linear interpolation is usually close enough for a first check.

Adjusting for Your Site

Data sheet values are declared at reference conditions. For reciprocating engines, the common reference is set by ISO 3046-1: 100 kPa total barometric pressure, 25°C air temperature and 30% relative humidity. Generator makers may state their own reference conditions, so check the data sheet’s footnotes.

Indian sites often differ:

• Hot ambient. A DG room in Delhi in June can see intake air well above 40°C. Hotter intake air is less dense, so the engine draws less air mass. At the same output, that tends to raise exhaust temperature. Some engines also derate at high temperature.
• Altitude. Sites in hill states draw thinner air, with similar effects.
• Hot DG rooms. Poor ventilation can push intake temperature far above outdoor ambient. Our guide to summer maintenance tips to prevent generator overheating covers room ventilation.

For most RECD work, these adjustments are modest compared with the uncertainty in load and condition. But for sets with tight backpressure margins, or in very hot rooms, ask the vendor to consider them.

Data at the Engine Outlet, Device at the End of a Pipe

The data sheet describes exhaust at the engine’s outlet. The RECD may sit several metres downstream, after the silencer, after bends and sometimes after a long horizontal run. Two things change along the way.

Temperature falls. Uninsulated pipe sheds heat, especially in a ventilated room or outdoors in winter. Exhaust that leaves the turbine at 400°C may arrive at a device ten metres away noticeably cooler. That matters for regeneration. Placing the device closer to the engine, or insulating the pipe, preserves heat.

Resistance accumulates. Every metre of pipe and every bend adds to backpressure. The data sheet’s maximum backpressure applies to the whole system measured at the engine outlet, so the RECD must fit within what the pipework and silencer leave. Our explainer on how exhaust backpressure affects a DG set shows how resistance adds up.

A good vendor uses OEM data at the engine outlet and then models the actual route. Ask to see that model.

When the Data Sheet Is Missing or Does Not Match

Not every set comes with a tidy data sheet. Common problems:

• Old engines whose data sheets are no longer published
• Rebadged sets where the generator brand differs from the engine maker
• Repowered sets where the engine has been replaced and the documents describe the old one
• Imported sets with data in unfamiliar units or for a different frequency and speed
• Modified engines with replacement turbochargers or altered fuel settings

What to do:

1. Identify the engine from its own nameplate, not the generator label, and ask the engine maker’s dealer for the data sheet by serial number.
2. Measure on site. A short load test with a calibrated thermocouple at the exhaust outlet, a manometer or pressure transducer at the backpressure point and a record of fuel consumption gives real figures. Many RECD vendors carry the instruments.
3. Estimate from first principles. Displacement, speed, aspiration and fuel consumption can produce a useful estimate. Our companion article on how engine displacement influences RECD selection walks through the method.
4. Combine sources. A measured temperature with an estimated flow is better than either guess alone.

Our guide on how to specify an RECD when original DG set documents are missing covers the paperwork side, and whether an RECD can be retrofitted to an imported diesel generator covers imported sets.

Putting OEM Data into Your RFQ

Here is a table to copy into your request for quotation. Fill in what you have; mark the rest “not available” rather than guessing. Ask every vendor to state the basis of any figure they use.

Item	Value	Unit	Source (data sheet page, measurement or estimate)
Engine make and model
Engine serial number
Rating type used (standby or prime)
Gross engine output at that rating		kWm
Rated speed		rpm
Aspiration
Number of exhaust outlets
Exhaust outlet diameter		mm
Exhaust flow at 100% load		state units and basis
Exhaust flow at 50% load
Exhaust flow at 25% load
Exhaust temperature at 100% load		°C, and measurement point
Exhaust temperature at 50% load		°C
Exhaust temperature at 25% load		°C
Maximum allowable backpressure		kPa, and measurement point
Existing system backpressure at full load, if measured		kPa
Fuel consumption at 100% load		L/h
Typical running load		% of rating
Reference conditions on the data sheet

Attach the data sheet itself. A vendor who receives the original can spot details that a summary misses.

What to Ask the Vendor About Their Use of Your Data

When proposals arrive, ask each vendor:

1. Which flow figure did you use, and on what basis: actual, normal or mass?
2. Which load point and rating column did you design for?
3. What exhaust temperature did you assume at the device inlet, after allowing for pipe losses?
4. What is the calculated pressure drop of the device when clean and at the recommended service point?
5. What is the total predicted backpressure of the system with the device, compared with the OEM limit?
6. How did you allow for our typical load when assessing regeneration?
7. Is the proposed device covered by a valid approval for our engine’s power? CPCB’s procedure for RECD compliance testing works with engine power ranges.

Clear answers to these questions are a good sign. Vague answers, or a reply that simply restates the kVA, suggest the device was chosen from a table rather than designed for your engine. Our guide to choosing CPCB approved RECD vendors and the article on how to read an RECD test certificate help with the wider evaluation.

Common Mistakes with OEM Exhaust Data

Mistake	Consequence	How to avoid it
Mixing actual and normal volumes	Device badly oversized or undersized	State the basis next to every figure
Using standby values for a prime-rated set, or the reverse	Wrong design flow	Match the column to your rating and duty
Using manifold temperature on a turbo engine	Overestimated regeneration temperature	Use turbine outlet temperature
Ignoring pipe heat loss	Overestimated temperature at the device	Allow for distance and insulation
Using only full-load data	Missed light-load regeneration risk	Look at the load point where the set actually runs
Taking backpressure limit as the device’s allowance	Total system exceeds the limit	Subtract existing system resistance first
Using generator brand data for a different engine	Data does not describe the real engine	Identify the engine from its own plate
Misreading backpressure units	Wrong budget	Convert everything to kPa

Our article on common RECD installation mistakes covers errors that appear later, during fitment.

Verifying the Data After Installation

OEM data is a starting point. After the RECD is fitted, commissioning should check that reality matches the design:

• Backpressure measured at the engine outlet at the highest practical load, compared with the OEM limit
• Exhaust temperature at the device inlet at typical load, compared with the design assumption
• Device readings on the RECD display or IoT system, recorded as a baseline

If measured backpressure is close to the limit, or inlet temperature is well below what was assumed, raise it with the vendor at once. Our RECD installation process guide places these checks in the wider commissioning sequence, and the RECD maintenance guide explains how to track them over time.

Frequently Asked Questions

Where can I find the exhaust flow for my generator?

On the engine maker’s performance data sheet for your engine model and rating, on the genset specification sheet, or in the installation manual. An authorised dealer can usually supply it from the engine serial number.

What is the difference between actual and normal cubic metres?

Actual cubic metres describe the exhaust at its real temperature and pressure. Normal cubic metres are corrected to 0°C and 1.013 bar. Hot exhaust in actual units can be roughly two to three times the normal figure, so always confirm the basis.

How do I convert exhaust flow from cfm to m³/h?

Multiply cfm by 1.699. Remember that the result is still at the same temperature and pressure as the original figure.

Which load point should the RECD be sized for?

Backpressure should be checked at the highest flow the engine will actually produce in service. Regeneration should be assessed at the load where the set spends most of its time. Both matter.

What if my engine’s data sheet is not available?

Identify the engine from its nameplate and ask the maker’s dealer. If that fails, measure exhaust temperature, backpressure and fuel consumption on site during a load test, and estimate flow from displacement and fuel data.

Is the maximum backpressure on the data sheet the allowance for the RECD alone?

No. It is the limit for the whole exhaust system. The silencer, pipework, bends and stack use part of it. The RECD must fit within what is left, including its resistance when partly loaded with soot.

The Data Sheet Is the Brief

The kVA rating tells a vendor the size of your generator. The exhaust section of the engine data sheet tells them what the RECD will actually face. Find it, read each field carefully, convert everything to one consistent basis, and put it in your RFQ alongside the original sheet. That single step removes much of the guesswork from RECD specification and makes competing quotes far easier to compare.

Aceget supplies CPCB type-approved RECDs for DG sets from 25 kVA to 2500 kVA and designs each installation from engine data and a site survey. For the size used in this article’s worked example, see the 200 kVA DG set RECD. To send your data sheet for review, contact Aceget or explore the retrofit emission control device range.