# Engine exhaust recovery: Science and thermodynamics

The boost trick behind record-breaking diesel efficiency

Source page: https://www.materialinsights.org/energy/e_engine_recovery/rc-sci/

What this tab covers The thermodynamics: the grade (temperature or pressure) of the energy being recovered, energy versus exergy efficiency, and how harsh the waste stream is on equipment.

A turbocharger uses hot exhaust to drive a compressor that packs more air into the cylinders; turbo-compounding adds a second turbine that returns exhaust energy to the crankshaft. Both tap exhaust at roughly 370-540 C, the only engine heat hot enough for mechanical recovery.

#### Grade of energy
Favourable

Engine exhaust is medium to high grade: DOE gives 700-1,000 F (about 370-540 C) for gas engines, while jacket water, oil and intercooler heat are below 300 F (about 150 C). Exhaust is also under pressure and moving fast, so a turbine can extract work directly without a separate steam cycle. Korlak's heat balance of large two-stroke marine engines confirms that exhaust is the only stream hot enough for turbines or steam; the rest suits only low-temperature ORC. In engines with exhaust gas recirculation, exhaust gas accounted for just 6.35-7.37% of heat losses, leaving less for any downstream recovery.^[3](https://betterbuildingssolutioncenter.energy.gov/sites/default/files/attachments/CHP_Reciprocating_Engines.pdf)^[4](https://yadda.icm.edu.pl/baztech/element/bwmeta1.element.baztech-756de647-bc5e-4a75-80b1-3c95ad960df5/c/Korlak_Comparative_2_2023.pdf)
 Gas engine exhaust temperature 700-1,000 F (about 370-540 C)^[3](https://betterbuildingssolutioncenter.energy.gov/sites/default/files/attachments/CHP_Reciprocating_Engines.pdf) Jacket water and oil heat below 300 F (about 150 C)^[3](https://betterbuildingssolutioncenter.energy.gov/sites/default/files/attachments/CHP_Reciprocating_Engines.pdf) Exhaust share of heat losses, EGR two-stroke engines 6.35-7.37%^[4](https://yadda.icm.edu.pl/baztech/element/bwmeta1.element.baztech-756de647-bc5e-4a75-80b1-3c95ad960df5/c/Korlak_Comparative_2_2023.pdf)

#### Exergy versus energy efficiency
Favourable

Turbochargers recover exhaust exergy as compressed air rather than electricity, which lets a smaller engine make the same power and is now standard on almost all diesel and large gas engines. The best engines show how much this matters: Weichai's 53.09% brake thermal efficiency heavy-duty diesel, shown in 2024, relies in part on an improved mixed-flow pressurisation (boost) system; US and European peers peak at about 46-48%. Turbo-compounding adds a few per cent more: Volvo quotes 7.5% better fuel efficiency on its D13TC, lifting brake thermal efficiency from 43% to 48% (company figures, 2018), and Detroit claimed up to 5% against its older Series 60 when the DD15 launched in 2007 (dated). Large two-stroke dual-fuel engines already exceed 50% efficiency. Accelleron, which describes itself as market leader in high-power turbochargers, says turbocharging can raise large-engine efficiency by up to 10% and claims its units are up to 2% more efficient than rivals (company claims).^[1](https://www.ttnews.com/articles/oems-mixed-turbo-compounding)^[2](https://www.truckinginfo.com/news/detroit-diesel-dd15-debuts-better-fuel-economy-performance-)^[4](https://yadda.icm.edu.pl/baztech/element/bwmeta1.element.baztech-756de647-bc5e-4a75-80b1-3c95ad960df5/c/Korlak_Comparative_2_2023.pdf)^[8](https://dieselnet.com/news/2024/04weichai.php)^[11](https://accelleron.com/content/dam/accelleronind/documents/investors/accelleron-annual-report-2025.pdf)
 Record heavy-duty diesel efficiency (2024) 53.09%^[8](https://dieselnet.com/news/2024/04weichai.php) Typical US and EU peak about 46-48%^[8](https://dieselnet.com/news/2024/04weichai.php) Volvo D13TC turbo-compound fuel gain 7.5%^[1](https://www.ttnews.com/articles/oems-mixed-turbo-compounding) Two-stroke dual-fuel engine efficiency above 50%^[4](https://yadda.icm.edu.pl/baztech/element/bwmeta1.element.baztech-756de647-bc5e-4a75-80b1-3c95ad960df5/c/Korlak_Comparative_2_2023.pdf) Large-engine efficiency gain from turbocharging (company claim) up to 10%^[11](https://accelleron.com/content/dam/accelleronind/documents/investors/accelleron-annual-report-2025.pdf) Volvo D13TC brake thermal efficiency (company) 43% to 48%^[1](https://www.ttnews.com/articles/oems-mixed-turbo-compounding)

#### Media compatibility and corrosiveness
Mixed

Turbines sit directly in raw exhaust, so blades and housings must tolerate high temperature, soot, ash from lubricating oil and, on ships burning heavy fuel, sulphur and vanadium deposits that require regular washing. The power turbine in a turbo-compound unit spins above 40,000 rpm at rated engine speed, so balance and bearing wear set service intervals. Exhaust aftertreatment (particulate filters and selective catalytic reduction) needs heat too, which competes with recovery for the same energy.^[2](https://www.truckinginfo.com/news/detroit-diesel-dd15-debuts-better-fuel-economy-performance-)^[3](https://betterbuildingssolutioncenter.energy.gov/sites/default/files/attachments/CHP_Reciprocating_Engines.pdf)
 Turbo-compound power turbine speed above 40,000 rpm^[2](https://www.truckinginfo.com/news/detroit-diesel-dd15-debuts-better-fuel-economy-performance-)

#### Variants compared^[1](https://www.ttnews.com/articles/oems-mixed-turbo-compounding)^[2](https://www.truckinginfo.com/news/detroit-diesel-dd15-debuts-better-fuel-economy-performance-)^[6](https://www.powermag.com/wrtsils-latest-combustion-engine-offers-new-options-for-intermediate-capacity)^[7](https://inis.iaea.org/records/0dcv3-mc375)^[11](https://accelleron.com/content/dam/accelleronind/documents/investors/accelleron-annual-report-2025.pdf)

| Variant | How exhaust energy is used | Typical gain | Where used |
|---|---|---|---|
| **Turbocharger** | Drives compressor for intake air | Up to 10% large-engine efficiency gain (Accelleron claim); enables downsizing | Almost all trucks, ships, gensets |
| **Mechanical turbo-compounding** | Second turbine geared to crankshaft | Up to 5% (DD15) to 7.5% (Volvo D13TC) fuel saving | Long-haul trucks (Volvo, Mack, Detroit DD16) |
| **Steam bottoming of engine plant** | Exhaust boiler and steam turbine | Engine plant efficiency from above 48% to above 50% | Large gas-engine power plants (Wartsila Flexicycle) |
| **ORC or Kalina on exhaust** | Organic or ammonia-water cycle | About 1.6 MW from 346 C exhaust of two 8.9 MW diesels | Studied for large engines; few installed |

#### Sources

1. [OEMs mixed on turbo compounding](https://www.ttnews.com/articles/oems-mixed-turbo-compounding) Transport Topics, 2018
2. [Detroit Diesel DD15 debuts with better fuel economy and performance (2007, dated)](https://www.truckinginfo.com/news/detroit-diesel-dd15-debuts-better-fuel-economy-performance-) Heavy Duty Trucking (truckinginfo.com), 2007
3. [Reciprocating Engines, CHP Technology Fact Sheet Series (DOE/EE-2764)](https://betterbuildingssolutioncenter.energy.gov/sites/default/files/attachments/CHP_Reciprocating_Engines.pdf) US Department of Energy, 2023
4. [Comparative analysis of the heat balance results of the selected Tier III-compliant gas-fuelled two-stroke main engines (Korlak), Combustion Engines 193(2)](https://yadda.icm.edu.pl/baztech/element/bwmeta1.element.baztech-756de647-bc5e-4a75-80b1-3c95ad960df5/c/Korlak_Comparative_2_2023.pdf) Combustion Engines (journal), 2023
5. [Wartsila's latest combustion engine offers new options for intermediate capacity](https://www.powermag.com/wrtsils-latest-combustion-engine-offers-new-options-for-intermediate-capacity) POWER Magazine, 2012
6. [Heat recovery from Diesel engines: a thermodynamic comparison between Kalina and ORC cycles, Applied Thermal Engineering 30(2-3), 212-219](https://inis.iaea.org/records/0dcv3-mc375) IAEA INIS record of journal article, 2010
7. [Weichai unveils diesel engine with 53.09% brake thermal efficiency](https://dieselnet.com/news/2024/04weichai.php) DieselNet, 2024
8. [Annual Report 2025 (company-reported figures and claims)](https://accelleron.com/content/dam/accelleronind/documents/investors/accelleron-annual-report-2025.pdf) Accelleron Industries, 2026

## Other sections

- [Summary](https://www.materialinsights.org/energy/e_engine_recovery/index.md)
- [Overview](https://www.materialinsights.org/energy/e_engine_recovery/overview/index.md)
- [Correlation](https://www.materialinsights.org/energy/e_engine_recovery/correlation/index.md)
- [Engineering and integration](https://www.materialinsights.org/energy/e_engine_recovery/rc-eng/index.md)
- [Economics and contracts](https://www.materialinsights.org/energy/e_engine_recovery/rc-econ/index.md)
- [Impact and additionality](https://www.materialinsights.org/energy/e_engine_recovery/rc-impact/index.md)
- [What to track](https://www.materialinsights.org/energy/e_engine_recovery/tracking/index.md)
- [Relationships](https://www.materialinsights.org/energy/e_engine_recovery/links/index.md)