Engine and A/C parasitic losses quantified

[Ecky]

The question I'm sure everyone has been wondering and nobody thought to answer.

My method:

Start the engine, let it idle until the coolant gauge *just* reaches full bars (coolant is not fully warm, but high idle has just finished, and the thermostat should be starting to open), then use LiControl to manually hold the engine at an exact RPM (+/- 50) as read via an OBD reader. Read LiBCM's "watts" to determine how many watts are needed to spin the rotating assembly in neutral. Record these values as "Engine Cold Watts".

Take a 45 minute drive, where I did not use much assist, to give the engine more opportunity to warm up. Repeat test and record these values as "Engine Warm Watts".

Repeat test with A/C turned on. Subtract warm RPM power usage to isolate A/C power usage.

~

-I believe the engine was likely still adding some small amount of fuel when "cold" at 1000rpm and 1500rpm, even though idle had settled below these, because the cold engine required less energy to spin at these RPM than the warm engine.

-Friction and parasitic losses continue to decrease even after the engine has reached full bars on the coolant gauge. This makes sense as, at the very least, the oil would not be at operating temperature.

-A/C compressor power consumption increases linearly with engine RPM, not exponentially. At 2000rpm (58mph / 90kph), the A/C is drawing 1900 watts, or 2.5 horsepower.

-Parasitic losses of a warm engine at 2000rpm (equivalent to 58mph / 90kph in 5th) are 3100 watts, or 4.2 horsepower.



Let's do some more math! And let's really butcher the units:

My past testing has shown a well sorted Insight can get around 85mpg at 58mph on flat, level ground, after the car is fully warm, with the A/C off. Traveling at 58mph / 2000rpm for one hour, at 85mpg, would consume 0.682 gallons.

The BSFC chart I have access to shows the Insight peaks right around 215g (gasoline) per kWh energy of usable energy extracted. Gasoline is roughly 0.74kg/L, or 2800g per gallon. So, the Insight engine gets 1kwh of usable energy out of 0.0768 gallons of gasoline, which contains 2.59kwh of total potential energy in its bonds. That gives us a rough thermal efficiency of (1/2.59) = 38.6% for the Insight's engine. For reference, Toyota's best beltless Atkinson cycle engine in 2025 is 40-41% thermally efficient.

If we assume 33.7kwh per gallon as a conversion factor, that's 23.0kw equivalent in gasoline (0.682 gallons/hour) to travel at 58mph. Multiply 23.0kw by our 38.6% efficiency, and we get 8.88kw, or 11.9 horsepower with the A/C off, or 14.4 horsepower with the A/C on. Or, in other words, at 58mph, the A/C consumes 17% of the energy needed to push the car down the road, or just under 15mpg penalty (85mpg -> 70mpg at 58mph).

Some more fun implications. If the engine's parasitic losses are 3100 watts out of our 8880 watts to travel at 58mph, that would suggest that, sans engine losses, the Insight only needs ~5800 watts or 7.75 horsepower to overcome rolling resistance, wind resistance, and non-engine drivetrain related frictional losses, all put together, at this speed.

[bandit86]

Use scanguage. Steady speed,con cruise control, hit AC button

[redpoint5]

I think I mostly followed. As bandit mentions, it seems the most straightforward way to measure AC fuel consumption is to note fuel consumption with the AC on, and then subtract the fuel consumption with the AC off.

Modern vehicles use PWM on the injectors to regulate fuel delivery, meaning the consumption can be known with a high degree of accuracy when the fuel pressure, injector duration, and RPM are also known.

[Ecky]

Originally Posted by redpoint5

I think I mostly followed. As bandit mentions, it seems the most straightforward way to measure AC fuel consumption is to note fuel consumption with the AC on, and then subtract the fuel consumption with the AC off.

Modern vehicles use PWM on the injectors to regulate fuel delivery, meaning the consumption can be known with a high degree of accuracy when the fuel pressure, injector duration, and RPM are also known.

The issue with measuring fuel consumption is that engine thermal efficiency varies widely across the map. You can correlate the compressor to fuel, but not directly to energy. I'm spinning the compressor with an electric motor and measuring directly the wattage drawn by the electric motor. I also have the ability to spin the engine with that electric motor, and measure the wattage directly. Which I've done.

[bandit86]

Originally Posted by Ecky

The issue with measuring fuel consumption is that engine thermal efficiency varies widely across the map. You can correlate the compressor to fuel, but not directly to energy. I'm spinning the compressor with an electric motor and measuring directly the wattage drawn by the electric motor. I also have the ability to spin the engine with that electric motor, and measure the wattage directly. Which I've done.

Yes, but in the same road, same time, same temp, the instant reading will show difference. And try at different speed as car at low rpm only spins compressors very little. Try at idle too

[Ecky]

Originally Posted by bandit86

Yes, but in the same road, same time, same temp, the instant reading will show difference. And try at different speed as car at low rpm only spins compressors very little. Try at idle too

Yep, it's in the graph.

[Logic]

Important to remember here is the fact that aero drag from open windows overtakes the parasitic losses of the aircon at around 120 km/h. (75mph)

[freebeard]

I've always wondered about this:

https://external-content.duckduckgo....ed0&ipo=images

It's got to be worse than a rear view mirror.

[Logic]

Originally Posted by Ecky

Some more fun implications. If the engine's parasitic losses are 3100 watts out of our 8880 watts to travel at 58mph, that would suggest that, sans engine losses, the Insight only needs ~5800 watts or 7.75 horsepower to overcome rolling resistance, wind resistance, and non-engine drivetrain related frictional losses, all put together, at this speed.

At 8kg for 10kw this range extender, made from a moped engine and CNC machine motor, will lighten the car up a bit!
Brake specific fuel consumption: 0,281 kg/kWh
https://sci-hub.ru/10.4271/2011-24-0085

Which begs the question: Why the big engines in serial hybrids!??

[Ecky]

Originally Posted by Logic

At 8kg for 10kw this range extender, made from a moped engine and CNC machine motor, will lighten the car up a bit!
Brake specific fuel consumption: 0,281 kg/kWh
https://sci-hub.ru/10.4271/2011-24-0085

Which begs the question: Why the big engines in serial hybrids!??

The 1.0L engine in the Insight weighs 58kg and has a brake specific fuel consumption of 215kg/kwh.

In the case of the Insight (a 26 year old car) the engine needed to primarily propel the car, most of the time. A smaller range extender might be used in modern vehicles that can primarily be driven on battery. It's certainly a question worth asking - do you put an engine in the car that's big enough, that the car can be propelled by it indefinitely, without running out of supplemental battery? Or do you pick some smaller size, such that the engine cannot keep up with the demands of cruise, but adds extra range? Do you build it to be highly efficient, or simply small and light?

The Insight's (26 year old) ICE is also 39% thermally efficient, compared with ~28% for the quoted engine. You get a roughly 24mpg or 10km/L improvement when cruising for an extra 50kg weigh penalty, with 26 year old tech.