Waste heat power flows of an IC engine


Of all the energy that is contained in the fuel that we consume in internal combustion engines, only a small part is transformed into power on the crankshaft. For the most optimal points of operation, modern diesel engines have an maximum efficiency of about 43% and gasoline engines of about 35%. The rest is lost. These losses go mainly to the exhaust and the engine cooling system, in both ways as heat. In everyday operating conditions the optimal efficiencies of an IC engine are hardly ever reached and automobiles can find an average efficiency on driving cycle of around 12%. Trucks are better adapted to get high efficiency when hauling on the motorway and are then often very close to 43%.

Though we also have done this exercise on a truck diesel engine, this evaluation of heat power flows is based on a mainstream gasoline 1600 cc engine.. Its Specific fuel consumption (SFC) diagram is as follows:

Specific Fuel Consumption diagram of a NA gasoline engine
figure 1 : Specific Fuel Consumption diagram

We would like to emphasize that in so many driving conditions the waste heat represent a real important part of the total power flow. We have made this clear in the following diagrams that were obtained with a 1600 cc gasoline engine on a testbench. The temperatures were measured after the catalyst.

exhaust temperatures of a NA gasoline engine
figure 2 : Exhaust temperatures of a gasoline engine

The power of the waste heat was then calculated by taking into account air mass flow. This yielded the following diagram:

The exhaust potential power is considered to be relative to the ambient temperature. It is calculated as :

Exhaust Thermal Power = Mass Flow . Cm . [ Available Temperature - Ambiant Temperature ]

It is to be noted that the exhaust gas temperature is increased in the catalyst by the catalyst’s conversion from chemical power to thermal power of the un-burned exhaust gas fractions (CO, HC).

Power in the exhaust of a gasoline engine
figure 3 : Power available in the exhaust

When comparing this power with the power available on the crankshaft we see that there is a part at the bottom of the diagram where more than 150% of crankshaft power is going through the exhaust. For the higher loads the power flows are about equal.

Power ratios of exhaust over crankshaft
figure 4 : Power ratios Exhaust vs Crankshaft

For the loads up to 40 Nm we made a zoom on the diagram above:

Power in the exhaust of a gasoline engine - zoom
figure 5 : Zoom on bottom part of Power ratios diagram

We can clearly see the correlation between the SFC diagram and the power ratios diagram: the SFC is poor when the ratio [exhaust / shaft] is high, that is at low loads, or at high RPM. This isn’t a surprise because at low loads the combustion is very slow, because the maximum pressure is low. And a slow combustion leads to a lot of power converted only once in the exhaust.

The heat2power concept has efficiencies of heatgeneration depending from engine speed and load. Especially the speed has an influence and tends to degrade the efficiency due to the drag losses around the valves above 4000 RPM. Our simulation models currently show the following power levels for heat regeneration:

Waste heat regeneration ratios
figure 6 : Regeneration ratios

From this diagram we multiply the values with the Powerflow in the exhaust and add the resulting Power to the power that is already on the crankshaft. While the fuel consumption is considered the same the sum of the two power flows will then result in the following Specific Fuel Consumption (SFC) diagram:

New specific fuel consumption diagram
figure 7 : New Specific Fuel Consumption diagram

The most important points for eveyday use of a passenger car are not related to the point of best efficiency. The most relevant points are those that correspond to the standardised driving cycle and to the points related to motorway driving. From litterature we have extracted a diagram that shows the distribution of occurrence of engine operation for a Porsche 6 speed engine/gearbox. The most used points of operation are around 2500 RPM and 20% load. Another area specifically related to high speed german motorway is around 5500 RPM and 95% load. The latter is however not relevant for most countries. For average European and American motorway driving this would be around 3000 RPM and 25% load.

Residency plot of gasoline engine on driving cycle
figure 8 : Residency plot of Porsche 911 with gasoline engine and AMT on Autobahn in Germany

The following diagram shows a more representative profile of engine use. It is from a 3.0 liter Cadillac Catera with 4 speed automatic transmission on the European driving cycle. For average European and American motorway driving the engine would run around 3000 RPM and 15-20% load.

Residency plot of 3 liter V6 gasoline engine on driving cycle
figure 9 : Residency plot of Cadillac V6 with Automatic transmission on driving cycle

If we look at the area of engine operation on the original and on the New SFC diagrams we can see the potential of waste heat recovery during every day driving. In the range of 1500-2500 ROM and 15-40Nm as delimited by the red rectangle in both diagrams the SFC can be lowered considerably:

Specific fuel consumption diagrams compared
figure 10 : Old and new Specific Fuel Consumption diagrams

The enthalpy contained in the exhaust gas of this engine is used as the heat source of the heat2power concept.