Hi
A bit out of date with thermal efficiency. Gasoline engines are around 36%
and diesel from 40% to nearly 60% for very large engines.
-----Original Message-----
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com]
On Behalf Of Eric
Sent: Wednesday, 4 August 2010 11:47 a.m.
To: electricboats@yahoogroups.com
Subject: [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.com, Grapeview Point Boat Works
<boat_works@...> 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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