However, with gas direct injection and all the advances in fuel injection I think it's a reasonable question to ask why nobody builds a gas tractor. I am wondering though, if the costs of such an advanced gas engine make it impractical. All of that technology, advancement in materials does not come cheap. I think a gas tractor would need a price advantage and then the market would be mostly small tractors I think. Guys that do 100 hours a year or less do not care so much about gallons/hour as the total fuel bill for the year is similar to what you pay making frequent stops at Starbucks for coffee.
I'm thinking a Kubota BX sized tractor that is battery/electric. Most guys with 2 acres or less could probably get most projects done with 2 hours of constant usage and then a charge. Totally quiet. Nice and heavy due to batteries, etc. Probably the hydraulics for a loader or backhoe would switch to linear actuators. Use for 2 hours until the battery meter shows low, have coffee and a nap, then go at it again.
A lot of diesels have been direct injected and turbocharged for many years. I don't see direct injection being cost prohibitive, almost every new car is now direct injected. I see the cost to be similar to what the pre tier 4 diesels cost. Direct injection is not really needed for tractors anyways, but it does have some good benefits. Turbocharging is where a gas engine will gain the most torque.
An electric tractor is definitely in the future, just too cost prohibitive right now and I doubt there would much interest in the market. The new electric cars are getting 250 miles out of a single charge. A comparable gas engined car would burn roughly 7-8 gallons of gas for 250 miles. Considering you could have a much larger battery on a tractor, you could easily get 4 hours of heavy run time out of an electric tractor. Just have a swappable battery pack and you could go all day!