As many as 10 teams have fallen by the wayside in the $10 million Progressive Insurance Automotive X Prize contest for ultra-efficient cars — and other teams are working hard to stay in.

When this week's second round of on-track testing began at the Michigan International Speedway in Brooklyn, Mich., 24 teams were in the running for the prize. The competition is aimed at encouraging the development of marketable cars capable of getting 100 miles per gallon of gasoline, or equivalent energy efficiency for other power systems. That's what's known as 100 MPGe.

During this first week of the Knockout phase, the challenge has been all about being able to hit the 67 MPGe mark, which is two-thirds of the efficiency goal. The cars also had to prove that they could go 67 miles (for two-seaters) or 134 miles (for four-seaters) without refueling. If they can do that, then they move on next week to further rounds of tests to check their emission levels and handling. Those tests are due to continue until June 30. The cars that satisfy the requirements will return to Michigan for an even more grueling round of finals next month.

The $10 million purse, put up by Progressive, will be split among the top finisher in the four-seater "mainstream" category ($5 million), plus the leading two-seater "alternative" teams in two categories (tandem and side-by-side, $2.5 million each). That's assuming that at least some of the cars attain 100 MPGe, while satisfying requirements for range, emissions and safety. A lot of teams are not going to make it. AutoblogGreen's Sebastian Blanco quoted the competition's senior director, Eric Cahill, as saying he thought 10 to 16 cars would qualify to move on to the finals.

The Knockout stage isn't even over yet, and Cahill's assessment has already come true. On Friday, before the week's standings were announced, Illuminati Motor Works team leader Kevin Smith told me "there'll be some surprises on the website." The results posted on Saturday morning weren't pretty.

Smith's team was one of the survivors, but he's not overjoyed to see fellow competitors fail. "We've got some friends on other teams," he said. "It's upsetting to see this."

Here are the teams that have been dealt setbacks so far in this round:

• American HyPower: Eliminated due to a faulty fuel sensor. In a Facebook posting, the team emphasized that their elimination was not related to their car's hydrogen-fueled engine. "Although we're no longer in the competition, we'll continue to follow it as we further develop our technology," the team said.

• BITW Technologies: The standings show that the Indiana-based BITW team's Vincitore 1000, a Chevy Metro that was modified to use a three-cylinder biodiesel engine, was eliminated this week.

• Cornell 100+ MPG: Withdrew just as the Knockout stage was getting under way, due to a problem with the electronics that monitor the car's battery power, according to Consumer Reports.

• Edison2: The Virginia-based team came to the Knockout stage with four cars. This week its side-by-side two-seater was eliminated, but the team still has two four-seaters and a tandem two-seater in the running.

• Enginer: X Prize organizers said the team's hybrid steam combustion/electric vehicle was eliminated.

• Global-E: One of its two high-efficiency prototypes, the Pulse, was eliminated before the Knockout phase began. Its G1 model didn't make the grade during this week's testing, according to the X Prize standings. That means the team is finished for this year.

• K-Way MOTUS: As I mentioned a few days ago, the Italy-based K-Way team had to withdraw from the competition due to engine troubles.

• Liberty Motors Group: Also withdrew as the Knockout phase was beginning.

• Optamotive: The side-by-side, all-electric E-Rex was eliminated, but the judges' decision was still being appealed as of Saturday.

• Team EVX: The Texas-based team's all-electric SmartCar was eliminated this week.

• West Philly Hybrid X: This high-school team was a sentimental favorite, but both of its hybrid cars were knocked out during this week's trials, X Prize spokeswoman Arron Robinson told me.

That's 12 cars eliminated this week, including the two from West Philly. Nine teams are no longer in the $10 million competition, one team is appealing the elimination, and Edison2 still has other cars in the race. Additional cars could be eliminated as the teams "move on to complete next week's acceleration, braking and avoidance maneuver tests conducted by Consumer Reports' staff," Liza Barth wrote Friday on Consumer Reports' blog.

On the plus side, Western Washington University's team reportedly aced the range test on Friday and will move on to the next round. ZAP Alias said its celebrity driver, Al Unser Jr., "congratulated the team" on Friday's performance. Canada-based FVT Racing said its hybrid electric/gasoline fuel-vapor vehicle passed the range test "with flying colours."

Based on the reports so far, Kevin Smith and his Illuminati team were among the week's big winners. Their swoopy-looking Seven vehicle reportedly averaged 119.8 MPGe in its efficiency tests. AutoblogGreen's Blanco said the Seven's batteries needed some conditioning to achieve the required range - and when I asked about that, Smith acknowledged that working with the new batteries was a challenge.

"It's still posing some challenges, but we're getting there," he said.

The Knockout tests aren't finished yet. Next week, the survivors will have to prove they can handle the road safely and operate within emission standards. (The all-electric Seven should have no problem with that latter issue.) And then there are the finals next month ... and the lab tests in August ... and the awards ceremony in September. The end of the road, and the definitive demonstration that 100-mile-a-gallon efficiency is technically possible as well as marketable, is still a couple of months away for the X Prize competitors. But as Smith said, they're getting there.

Update for 9:30 p.m. ET June 28: There's more bad news from Monday's acceleration tests. Consumer Reports' Jeff Bartlett says via Twitter that the German-made, battery-electric TW4XP two-seater tried to pass its acceleration test and "did not make it." Bartlett also said Illuminati's Seven didn't make the grade: "Team Illuminati did not pass the accel test. Painful to see. Team has much to be proud of."

If Illuminati is out of the running, then the $5 million prize for the most efficient four-seat mainstream car would be Edison2's to lose. The only mainstream vehicles still in the competition are the two Very Light Cars fielded by Edison2. The same team has a two-seater in the running for an alternative-category prize. Edison2 reported that it exceeded the required 67 MPGe standard and hit 100 MPGe during the Knockout efficiency tests.

Over the weekend, I spoke with Eric Cahill, the competition's senior director, and he told me that the 67 MPGe requirement "has been the biggest hurdle" during the Knockout stage. But it sounds as if the zero-to-60 test is a killer as well. Cahill was well aware that the challenge for four-passenger vehicles is tougher than it is for two-passenger vehicles.

"We are very optimistic that we are going to have winners, most certainly in the alternative [category]," he told me. "With the mainstream [category], we're optimistic - but it's going to be a bit narrower."

Update for 12:30 p.m. ET June 29: In today's blog post, Consumer Reports' Jeff Bartlett indicates that TW4XP was at last able to pass the acceleration test. Illuminati appears to be out of the competition, along with the team fielding the Amp Electric Sky sports car. But Bartlett points out that appeals are still being heard, and so the word on some of these eliminations is not yet definitive. Stay tuned for the semifinal answers. (The finals are scheduled next month.)

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