Transonic Combustion, a developer of fuel injection technology for internal combustion engines, raised $32 million in Round D venture funding from Venrock, Khosla Ventures, Rustic Canyon and Saints Capital, according to Dan Primack at Fortune.

According to the company's website, injecting supercritical fuel directly into the combustion chamber enables improvements in efficiency. The company states that "supercritical injection enables cost-effective compression ignition of gasoline in engines with a conventional architecture. This is described as “Injection Ignition,” and it results in efficiencies that are equal to or better than today’s Diesel engines."  

Despite the attention paid to electric cars like Tesla and the Volt or hybrids like the Prius or Fisker, startups and some of the major carmakers are focused on better gas mileage. Surely one way to ratchet back petroleum usage is to double miles per gallon performance.

Firms like Achates Power and EcoMotors have developed dual compression/dual piston engines. These companies claim they can achieve 100 miles per gallon. Ford has an Escort in Europe which gets 65 miles a gallon on diesel. Scuderi has a new engine technology, as does Zajac Motors. Other startups are working on batteries or capacitors for emerging "microhybrids."

With a CAFE standard for vehicle fleets of 54.5 mpg by 2025, many automotive and battery startups -- Scuderi, EcoMotors, Achates Power, WrightSpeed, Nanostellar, PowerGenix, etc. -- hope to license their technologies for energy-efficient engines and other components to major car makers. Carmakers are reluctant to license, and selling to the automotive sector is always a challenge -- even more so for a cash- and time-strapped VC funded startup. But it's either build your own vehicle, or work with the majors.

Here are some technologies looking to get to better gas mileage:

1. Diesel: Diesel engines generally get better mileage than their gas counterparts and auto manufacturers have managed to cut down the particulate matter, SOx and NOx, that gave diesel a bad name in the '70s, '80s and '90s. Nearly half of all U.S. gas stations have diesel now too, Audi execs tell us. 

2. Electrics and Plug-In Hybrids: Toyota, Ford, Volkswagen, Honda and most other manufacturers all have electrics coming. Renault-Nissan CEO Carlos Ghosn has said electrics will constitute 10 percent of industry shipments by 2020 -- 85 million cars will be sold and 8.5 million will be electric, he claims. Tesla Motors CEO Elon Musk has said electrics could account for 12 percent or so of vehicle sales by 2020. Cynics put the figure in single digits, warning of evaporating incentives.

3. Regular Hybrids: Ford execs predict that 10 percent to 25 percent of their cars will be electrics, plug-ins or hybrids by 2020. But the vast majority will be regular hybrids.

4. Stop-Start:

Also called microhybrids, these cars are like regular hybrids but they have very small electric motors that engage mostly to allow a car to get started quickly. With a micro, a gas engine can go to sleep at an intersection. A microhybrid system can improve gas mileage by 6 percent to 10 percent, according to Dan Squiller, former CEO of battery maker PowerGenix.

5. Waste Heat: Approximately 50 percent of all energy purchased gets lost as waste heat, according to UC Berkeley. Engines are particularly attractive sources for waste heat: some estimate that 85 percent of the energy injected into engines gets wasted. That heat can be captured, compressed and then exploited to run the air conditioning system. Panasonic has developed a waste heat AC system for cars: it is derived from a household AC unit. (See video here.) Tempronics, Phononic Devices and Alphabet Energy are also working on devices for converting waste heat into electricity. 

6. New Engines:

Pinnacle Engines is working on an opposed-piston gas engine, while EcoMotors and Achates Power have opposed-piston diesels. Pinnacle says it can improve fuel consumption by 25 percent to 50 percent. Opposed piston engines were used in aircraft in WW II, but have been refined and retrofitted now for cars. These engines also require fewer raw materials like steel.

Expect to see new, improved engines from the established players. Ford is expanding the number of models of cars it makes that will come with its EcoBoost engines. GM and Toyota have worked on HCCI engines for years. (Include Transonic Combustion in the startups-with-mileage-tech-to-license category.)

7. Plastics and Carbon: 

Weight. It's the third fuel. If you drop weight, mileage goes up. Bright Automotive, a startup that spun out of the Rocky Mountain Institute, is working on light delivery trucks. GM is an investor. Non-metallic bodies can also be curved for great aerodynamics. Aluminum will come first along with other lightweight alloys. 

8. Hemp:

Nearly every automaker is tinkering with ways to replace traditional car materials -- aluminum, rubber, etc. -- with sustainable ones like old plastic and hemp. Ford and others already exploit soy and other renewables.  

9. Innovative Transmissions

: Fallbrook Technologies pulled its IPO early last year, but its invention, the NuVinci, is still intriguing: a highly variable transmission that relies on balls instead of gears. The company claims it can improve mileage by 12 percent to 15 percent.

10. Hydrogenand Fuels Cells

. Honda and other automakers keep pushing out the deadline for hydrogen cars, and for good reason. Hydrogen is dirty to make, you can't transport it easily and the cars cost a ton of money. Honda, Mercedes, and others continue to research the concept. Maybe by 2025?