Google Answers Logo
View Question
Q: Small light engines needed ( Answered 5 out of 5 stars,   3 Comments )
Subject: Small light engines needed
Category: Science > Technology
Asked by: larry95-ga
List Price: $41.00
Posted: 25 Jul 2002 00:03 PDT
Expires: 24 Aug 2002 00:03 PDT
Question ID: 44907
I'm looking for small gas engines that are about 5-20 horsepower and
very light (pound or so per HP).  I'd prefer engines that are more
reliable and have less emissions (4 cycle better than 2, rather not
have gas/oil mixed fuel).  Prototypes are interesting, but engines
that are for sale are more interesting.  Engines that can last more
than a few hundred hours are also important (stock model airplane
engines are too light duty).

Please produce a chart with HP/pound ratios for some reasonable
alternatives.  Extra credit for any that can run on hydrogen rather
than gas.

Request for Question Clarification by chromedome-ga on 01 Aug 2002 14:24 PDT
What sort of duty cycle are you projecting for these engines?  What
sort of load will they be operating under? (ie, stationary use, light
vehicle, etc?)

Clarification of Question by larry95-ga on 01 Aug 2002 18:54 PDT
They only need to be able to provide maximum power for a couple of
minutes.  Then they will operate for up to a couple hours at a time at
reduced power.

Request for Question Clarification by chromedome-ga on 02 Aug 2002 13:02 PDT
One further point, and I'll let you get on with your weekend:  will
the application be ongoing (daily use) or sporadic (demonstration

I'd neglected to consider, in my comment on the battery question, that
not only is it now a weekend, it's a long weekend in Canada.  This
means that some of my local gurus may not be available to me until
Tuesday, but I may be able to look after both of these for you on my
own resources.

Subject: Re: Small light engines needed
Answered By: chromedome-ga on 06 Aug 2002 17:56 PDT
Rated:5 out of 5 stars
Hello again, Larry95!

While there are any number of engine manufacturers on the planet,
finding this unique combination of characteristics has proven rather
difficult.  Of course you already knew this, or you wouldn't have
turned to us.

I have interpreted the power/weight ratio and horsepower range as the
more important factors in your question, and the rest as preferences. 
I've included some engines above the 20hp range, as it seems you have
some flexibility, but if necessary they could likely be tweaked to
provide less power.

After viewing the websites of a great many manufacturers, I am unable
to locate a production four-stroke engine in the size and
power-to-weight ratio you want.  If anybody has one, they're being
oddly secretive about it.  Honda's miniscule GXH50/GXV50, for example,
puts out 2.5 horsepower, but at a weight greater than 5kg/11 lbs. 
This holds true for engines of larger output as well.  Honda's GX620
model, puts out 18hp at a weight of 41kg.  While I've used Honda for
my examples, I've compared model-to-model with engines from
Robin/Subaru, Briggs and Stratten, and numerous other manufacturers
and found much the same result.  The ratio seems to be in the range of
2kg/hp, which is roughly four times what you'd wanted.  I will not
post links to these manufacturers here, as none of them offer what you
want, but will do so in a clarification if you need that info for
reference purposes.

With two-stroke engines, the normal power to weight ratio is roughly
1kg/hp, or about twice what you are looking for.  However, I have
found a few candidates for you.  It would seem that the engines best
suited to your specification would be the lightweight, high output
models used in kart racing and ultralight aircraft.

The Rotax 100 KART PP-E model meets your 20hp limit, and just fails to
meet the 1:1 ratio of power/weight.  At the upper end, the 100 KART
DS-F-L/LC version of this engine is over your horsepower limit
(roughly 28hp) but exceeds the 1:1 ratio.  To view the list, click the
following link:

Note that Rotax engines are widely used in snowmobiles and personal
watercraft, so their parts & service network is excellent.  They are
part of the Bombardier "empire".

The Hirth Aero F33 is slightly above your hp requirement at 28hp, but
weighs only 35 lbs with exhaust system.

Perhaps the best of the bunch, for power-to-weight, is the
Swedish-made "Raket 120".  This engine is widely used both in karting
and ultralights, and at 14hp/15 lbs, it is remarkably close to that
1:1 ratio.  For the Raket engine, look here:

And for a grid comparing the Raket, Hirth F33, and many other engines
for ultralights, look here:

For your convenience, I have tabulated these models for you.  The
formatting in Google Answers is sometimes unpredictable, but this
should work if you cut-and-paste into your word processor, then choose
a monospaced font such as Courier.

Manufacturer        Model            HP        Weight (Approx)

Rotax               100KART PP-E     20        12 kg/26 lbs
Rotax               100KART DSFLLC   28        12 KG/26 lbs
Hirth Aero          F33              28        35 lbs
Raket               Raket 120        14        15 lbs

For a somewhat less favourable power to weight ratio, but smaller
engines, you may wish to consider chainsaw engines.  These typically
fall into the usual 1:2 power/weight ratio of two-stroke engines, but
are easily modified to reduce their weight.  This is a common practice
within the scale-model community, since (as you've observed) the
engines manufactured for model aircraft are not especially robust. 
Chainsaw engines, in contrast, are designed to operate for many hours
at a time with frequent revving.

I will not provide you with a comparison of major chainsaw makers,
because they all seem to be pretty close for power and weight. 
However, for extensive discussion of the work involved in modifying
this class of engine, I will refer you to this page:

There are a few newer technologies on the horizon which I'll mention,
since you'd expressed an interest in prototypes.

The ball-piston engine is a proposed technology (it does not appear to
have reached the prototype stage as yet) which would use conventional
four-stroke technology in a manner more akin to a rotary engine.  For
a detailed description of the design, look here:

The "Quasiturbine", or "Qurbine", is another high-efficiency engine
which appears to meet with more of your desired characteristics. 
Research prototypes are available for  rental in a variety of sizes,
including one aimed at the chainsaw market.  Models intended for
hydrogen and natural gas are in the works, as well.  If you wish to
bring a product based on this engine to market, it is possible that
either the Quebec or Canadian government might offer some incentives
to smooth the path.  Their home page (ugly, but lots of good stuff) is

Finally, the Wankel heritage is being brought into the new century by
Freedom Motors, of Davis CA.  Their designs are extremely
low-emission, and again can be hydrogen-fuelled. With only two moving
parts, their long-term reliability should be good.  Although this
article at NASA's website describes a miniscule 10hp engine,  
unfortunately that was a one-off test unit.  For now, the company is
focused on their Rotapower 530 engine, which will go into small
recreational vehicles such as ATV's and Personal Water Craft. 
However, in his reply to my e-mailed inquiry, general manager Bruce
Calkins indicated that they could readily produce an engine in the
size and power range you wish, if a suitable buyer should arrive at
their doorstep with financing in hand.  Their website is here:

Search strategy:

My initial search used these keywords, the latter two being added to
filter the results after some initial digging:

+lightweight +engine +gasoline -car -automobile +"4-stroke" -honda

Further searches used these keywords:

"high power to weight ratio"
+ultralight +engine
+chainsaw +engine
+kart +engine

I also searched on individual manufacturers, some of which turned up
in the course of my searching, and others (such as Honda and Briggs &
Stratten) that I already knew.

Thank you for an interesting question, and hopefully I'll have answers
for you on the other two within the next few days (awaiting return
e-mails at present).

If I've inadvertantly left anything out, or been unclear, please let
me know using the clarification function.

larry95-ga rated this answer:5 out of 5 stars
Good answer, most things I already knew, but a good summary.

Subject: Re: Small light engines needed
From: masdes-ga on 31 Aug 2002 13:13 PDT
A further source of small light engines are Model aeroengines.
they offer powers of factional HP to 4 or more, at very good power to
weight ratios. They run on a methanol+oil mix which is from renewble
sources and so better for the environment. Most  Model shops, around
the world stock many varieties of these a typical web based source
would be ..
Subject: Re: Small light engines needed
From: usatracy-ga on 14 Jun 2004 21:37 PDT
I would also advise you to look at military surplus, in particular the
4 stroke 4A032 4 cyl engine. 45 pounds stripped and 20 to 30 hp.
Subject: Re: Small light engines needed
From: yfzpilot-ga on 04 Feb 2005 14:34 PST

Important Disclaimer: Answers and comments provided on Google Answers are general information, and are not intended to substitute for informed professional medical, psychiatric, psychological, tax, legal, investment, accounting, or other professional advice. Google does not endorse, and expressly disclaims liability for any product, manufacturer, distributor, service or service provider mentioned or any opinion expressed in answers or comments. Please read carefully the Google Answers Terms of Service.

If you feel that you have found inappropriate content, please let us know by emailing us at with the question ID listed above. Thank you.
Search Google Answers for
Google Answers  

Google Home - Answers FAQ - Terms of Service - Privacy Policy