Piston vs. Gas Turbine Engines, Power/Size Ratios

When it comes to power/size and power/weight ratios, gas turbines are the undisputed winners over piston engines.  Tanks, heavy lift helicopters and high capacity portable power plants for industry are applications where having the highest possible power/size and/or power/weight ratio is a must.



The M1A1 Abrams uses a Honeywell AGT1500 gas turbine engine [1]

 

Robinson Helicopter Co. R66 gas turbine [2]



Solar Turbines, GS-350, 225 kW generator set [3]
Note: the generator is bigger than the turbine!

The first question that needs to be answered is, what are the size and weight constraints that the design of our robot harvester/tender puts on a possible choice for its power source?   We can get these design constraints by remembering that every design problem always brings with it its own size and weight scales.  So as long as the size and weight of our 'bot's power source is less that 5-10% of its overall size and weight, there is going to be no marginal benefit to making it smaller.

The target size and weight for our 'bot, that we estimated previously, was on the order of 6-8 ft long and less than 300-400 lbs.  This puts a size limit of 1-2 cubic-ft and 10-20 lbs on our 1-2 kW power source.

Here is a 1.3 HP gas engine from Honda that fits our design constraints easily.

Honda GX35, commercial lawn and garden engine [4]

Compared with a small gas turbine from BladonJets that also fits our design constraints. 

Bladon Jets, Micro Gas Turbine Engine [5]

So it seems that either choice of power source, piston or gas turbine, will easily fit into our robot harvester/tender's size and weight design constraints.

And just for comparison, here are a few spec's for a comparable reformed methanol fuel cell.
UltraCell's XX55: 50 W, 3.5 lb, 12.3" 3.2" 8.6" l/w/h

[1] M1A1 AGT1500 spec's
[2] Robinson Helicopter Co. R66's spec's
[3] Solar Turbines
[4] GX35 spec's: 1.0 kW, 7.6 lb, 8" 9.2" 9.4" l/w/h
[5] Bladon Jets