Automotive HP and torque at cruising speeds

[tpenfield]

Not entirely boating related, so I am posting this in the non-boating tech section . . .

I have always been curious on how much Horsepower and Torque a car generally takes to drive down the road. Folklore has it as 'not that much' as compared to what most engines are capable of.

My mid-size car has a 'sport display' that shows the instantaneous HP and torque as you drive. Not sure what is used as the sensor . . . maybe the torque converter :noidea:

So, as I was driving today, I thought to take a few pics of the readings as I drove.

Back road . . . about 40 mph

.
Looks like 55 hp and 175 ft-lbs of torque

Going on highway @ 60 mph . . .


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25 HP and 80 ft-lbs of torque.

It would be interesting to compare these readings to some typical readings of the hp and torque required to propel a 'mid-sized' boat :noidea:

 

[Scott Danforth]

most vehicles only need between 5hp and 20hp to push them down the road at 65mph. put really skinny tires on them to lower the rolling resistance and you can even get by with less power.

your car uses fuel burn rate (fuel pressure and injector PW), engine RPM and feed back from the knock sensors and MAP to estimate your current Torque, and ultimately calculate the HP

however to compare a car to a boat, load the car with sand, and drive up-hill in sand....... you will find that you are pushing nearly full HP to get out of the hole, then it drops down a bit when you get on plane.......then you want to go faster, and the power level goes up until the power required to move the boat matches all the motor(s) have for power.

here is a typical marine prop curve (stolen from the interweb). you can see on this cummins motor, the typical prop loading curve and the motor torque curve

https://i.imgur.com/M4fyonh.jpg

here is another tidbit.......

the radiator in your car is good to cool about 30% of your motors output. if you were to command full HP for any significant amount of time, you would overheat. most cars on a good day have a 10% duty cycle, and the system volume and the thermostat helps even things out. if you take the same engine and cooling system from any vehicle and put it in a 100% duty cycle, you would over-heat until you go up about 3X the square area of the radiator core. I used the same 24 x 24 x 4" core radiators for a 100kw marine genset as I did in front of a supercharged northstar in the Opel GT or the built 455 olds in my 1948 5-window pickup.

 

[tpenfield]

Interesting chart, Scott Danforth

As many of us know the boat engines seem to be using 100's of horsepower at cruising speeds vs. 10-20 hp for cars.

I have always thought of fuel consumption as a good way to make comparisons between cars & boats as a general indication of power usage.

So, a car getting 30 mpg at 'cruising speed' and using 15 hp vs. a boat getting 3 mpg, probably means it is using about 150 hp at 'cruising speed'.

Not sure if there have been more scientific comparisons. :noidea:

 

[Scott Danforth]

Brake Specific Fuel Consumption (BSFC) is .3333 # of fuel per HP per hr for diesel engines

this is how much fuel is burned to make power. on a turbocharged diesel the BSFC curve is basically flat

for Spark Ignited motors, BSFC is about 0.4 # of fuel per HP per hr. the curve on spark ignited is more of a contour map affected more by atmospheric conditions and load. however for a really good rule of thumb, assume its flat-ish

older 2-strokes were about 0.5 # of fuel per HP per hr.

so for 150 hp "Cruising" for 1 hour..... (150 x 0.4 x 1) / (6.82#/gallon) is 8.8 gallons. so assume you were at 30mph (covering 30.0 miles in the 1.0 hour), your fuel economy would be 3.4 GPM

however realistically your cruising at 220hp in your boat ( 220 * 0.4 x 1) / 6.82 would be 12.9 gallons or at 30mph, your fuel economy would be 2.32 GPM

but...... loading changes in a boat more so than on a highway. the water is dynamic, so small changes in current or direction will load or unload the motor a bit. adjusting trim tabs and drive trim will also change the loading.

 

[tpenfield]

I'd hate to see what 1.4 mpg looks like I think that is top efficiency for my boat . . . maybe cranking out 500 hp between the 2 engines . . . :noidea:

 

[dingbat]

I'd hate to see what 1.4 mpg looks like

I’m getting 1.9 mpg on flat seas.

Have seen 0.9 mpg in slop

 

[Scott Danforth]

I'd hate to see what 1.4 mpg looks like I think that is top efficiency for my boat . . . maybe cranking out 500 hp between the 2 engines . . . :noidea:

Going to see if my burn rate is 1.85 gallons per mile on Sunday

 

[achris]

I wish you guys wouldn't use 'gallons'... Because, which particular 'gallon' are you using? To me, a gallon is 4.55 litres...

Automotive fuel consumption is measured in litres/100 kilometres, and an average mid-sized car uses about 6 litres/100 kilometres cruising. To use the same units in my 'mid-sized' boat, cruising, about 67.5 litres/100 kilometres.

Chris.....

 

[Scott Danforth]

I wish you guys wouldn't use 'gallons'... Because, which particular 'gallon' are you using? To me, a gallon is 4.55 litres...

Automotive fuel consumption is measured in litres/100 kilometres, and an average mid-sized car uses about 6 litres/100 kilometres cruising. To use the same units in my 'mid-sized' boat, cruising, about 67.5 litres/100 kilometres.

Chris.....

ok, Im going to see if I can burn 220# of fuel per hour