T&T: AC genset discussion

[LRZeitlin at aol.com]

In a message dated 8/10/08 12:00:50 AM, R. Barber writes:


> This whole problem of having to run a generator engine at 1800 RPM, or
> twice that, in order to maintain 60 cycle current is exactly why I have
chosen to
> go to a system using a large 24 volt battery bank, a small 1 cylinder diesel
> DC genset, and a couple of 3600 watt inverters to supply AC house power.
> It is difficult for me to understand how one runs a 4 cylinder diesel at
> 1800 RPM with little  or no load economically.  There may very well be a
direct
> relationship between fuel consumption and load, but it seems like that
> relationship would break down at light or no load. 
>
> I hope we get a good discussion going on this subject, in that it is one
> I've struggled with a long time.  My comments do not address your question,
I
> know, but maybe someone else has a solution.
>

Well, I guess we got a lot of discussion on this topic. Here is my $.02.

Using a small DC generator to charge a big bank of batteries which power a
pair of large inverters seems to be a complicated process if what you want is
60
cycle AC power. And I'm not sure if the price will be any lower or the
efficiency any higher than installing a conventional AC genset. Small DC
generators
have efficiencies in the 85% range. Batteries can recover about 90% of the
charging current. A good AC inverter is also about 90% efficient. Multiplying
these efficiencies together and you find that a typical DC to battery to
inverter
to AC supply is only about 69% efficient. Basically you waste 31% of the fuel
driving the engine powering the electrical supply. A standard AC genset,
using the Perkins 4-107 engine described isn't any paragon of efficiency
either
but I suspect that it is higher than the more complicated DC to AC system.

Now about the Perkins 4-107/8 engine. Most gensets using this engine are also
set up to use the engine as a wing engine for single engine trawlers. It will
do an adequate job as an emergency propulsion engine for boats up to 60 feet.
Further the engine will last almost indefinitely at 1800 rpm. This seems to
be a sweet spot in the rpm range for this engine. It uses relatively little
fuel and has low vibration.

Perkins 4-107 engines were the stock engines supplied with Willard 30
trawlers. A survey of the Willard list showed that most owners cruised at 1800
rpm.
Mine has lasted since 1974 without ever having the head removed. No mystery
since the engine was originally designed to power London taxicabs which
regularly
run half a million miles between overhauls.

My own preference, admittedly for a smaller boat, is to use the engine
alternator to charge a modest bank of deep discharge house batteries. I have a
40
watt bank of solar cells to keep the charge up when the boat is idle or at
anchor. Most boat equipment runs on 12v. DC. For the occasional times I need
AC I
use dedicated inverters, up to 300 watts. I do have an AC genset aboard. It is
a 30 year old Sears 1200 watt gasoline powered camping generator which I keep
in a back deck compartment. No problem with the gasoline since I need a supply
for the dinghy outboard.

Larry Z


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