----- Original Message -----From: EricSent: Tuesday, August 03, 2010 7:46 PMSubject: [Electric Boats] Re: hull design"A gallon of gas is about 33KWh of stored energy" is my round number conversion of the chemical energy stored in gasoline starting with "32 MJ/liter (LHV)". (ORNL, n.d.)
According to Oak Ridge National Laboratory, "Energy contents are expressed here as Lower Heating Value (LHV) unless otherwise stated (this is closest to the actual energy yield in most cases). Higher Heating Value (HHV, including condensation of combustion products) is greater by between 5% (in the case of coal) and 10% (for natural gas), depending mainly on the hydrogen content of the fuel. For most biomass feedstocks this difference appears to be 6-7%. The appropriateness of using LHV or HHV when comparing fuels, calculating thermal efficiencies, etc. really depends upon the application. For stationary combustion where exhaust gases are cooled before discharging (e.g. power stations), HHV is more appropriate. Where no attempt is made to extract useful work from hot exhaust gases (e.g. motor vehicles), the LHV is more suitable. In practice, many European publications report LHV, whereas North American publications use HHV."
So you may see other numbers quoted for gasoline's energy density, but I attribute that to different measuring standards.
Gasoline engines typically operate at a thermal efficiency of 20-30%. A hyper-efficient engine at an optimum setting would do better and an engine at lower throttle will do worse. I usually use 25% thermal efficiency in my estimations of gasoline engines because that seems like a reasonable average.
Since I've now got access to some notes that I left in my office, here's one last conversion to put my calcs into normal terms. Using GPH*Thermal Efficiency/0.0226=HP we get the following:
"burning 0.20 GPH with an ICE engine" = 2.2HP (at the crankshaft).
Checking that result, 2000W times 80% thermal efficiency for electric motor equals 1600W or 2.13HP at the shaft, definately within rounding errors and my broad assumptions.
Here's the other estimates from my earlier post.
"0.25 GPH with an ICE engine" = 2.7HP, 2450W*0.80/750W per HP = 2.61HP
"0.6 GPH with an ICE engine" = 6.6HP, 6000W*0.80/750 = 6.4HP
These estimates assume that the motor and engine are attached to similar drivelines, i.e. inboard shaft, outboard, prop size, etc. to keep the other driveline losses and efficiencies equal.
While I understand that a lot of this sounds like I want to be a "rocket scientist", I'm just trying to find consistant ways to translate our conversion requirements into normal terms.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups. , Grapeview Point Boat Works <boat_works@.com ..> wrote:
>
> "A gallon of gas is about 33KWh of stored energy"
>
> Does this already account for the considerable losses of converting the gas into HP?
>
> -Tom
>
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