GM patent application shows how a longitudinal variable-compression engine could be made easier to service

General Motors has filed a US patent application for a longitudinal variable-compression engine layout intended to be simpler to service, with the document published after being spotted by CarMoses. The filing describes replacing the centre link in GM's earlier variable-compression design with a two-piece assembly built around the crankshaft.

GM holds numerous patents on applying variable-compression technology to car engines, and this application is the latest step in packaging the fuel-saving hardware for engines mounted lengthways rather than transversely. Nothing in the filing confirms a production engine, and GM has not announced which vehicles would use it.

How the two-piece design works

In the earlier GM patent, compression variability of roughly 7:1 to 11:1 came from a special connecting-rod linkage: the connecting rod attaches to one end of a centre link mounted on the crankshaft, and the other end of that link connects through a control rod to a control shaft. A phaser adjusts the control shaft's rotational timing relative to the crankshaft, which governs piston movement and varies compression.

The new application would swap that centre link for a two-piece assembly assembled around the crankshaft, with one part connecting to the piston rod and the other to the control rod. The bolts joining the two halves would be reachable from below the engine, likely with the oil pan removed. GM also says the design would allow conventional lubrication of the connecting rods, which together with the two-piece link could make the engine easier to service and less prone to bearing-related problems.

That matters because service access is one of the practical hurdles for variable-compression hardware. An engine that needs specialist attention deep inside its bottom end is a harder sell to fleets and high-mileage truck buyers than one a technician can strip from underneath.

Why the control shaft sits above the crankshaft

GM's design places the control shaft above the crankshaft, which the filing says frees space below a longitudinally mounted engine for engine mounts or for the vehicle's steering or four-wheel-drive equipment. That packaging question is central to longitudinal applications, where the engine sits front-to-back and the space underneath is contested by driveline and chassis parts.

GM's arrangement would also let the expansion stroke run longer than the compression stroke, by up to 40 percent, which carries potential efficiency benefits. Nissan's VC-Turbo, already on sale, achieves its Atkinson-cycle effect through valve timing instead, and it is designed for transverse front- and all-wheel-drive applications. Honda has commercialised a comparable extended-expansion concept called EXlink in household cogeneration units that supply electricity and hot water.

What the drawings show, and what they do not

The patent drawings depict a petrol inline-six, a layout GM does not currently use, but the filing is not evidence that such an engine is planned. The same concept could be applied to engines such as the L3B 2.7-litre four-cylinder, the unit with sliding cam sleeves used in Chevrolet and GMC half-ton trucks, which could also find its way into a new Camaro or Buick sedan.

GM's record supports reading the filing as more than paperwork: the company has brought its own versions of cylinder deactivation and sliding camshafts to production, and is currently enlarging and boosting its truck V-8s without entirely sacrificing efficiency. Variable compression would be the next fuel-saving technology to move from that pipeline into a wider range of engines, if the patent work translates into hardware.

The catch for buyers

A patent application is not a product announcement. No timeline, no confirmed engine family and no vehicle has been named, so drivers should treat the serviceability claim as an engineering intention rather than a specification. The detail worth watching is the two-piece link: if it reaches production, routine work on a variable-compression engine could look much like work on a conventional one, which is the difference between a niche technology and one that can be sold across a truck range.

General Motors has been granted a US patent for a launch control system that watches a drag strip's staging lights and launches the car automatically. US Patent 12,780,203, granted on November 3, covers a camera-based setup that predicts the green light and triggers the launch without the driver touching a pedal.

The patent, titled "Methods and Systems for Coordinating Launch Control for Vehicles for Racing," was first filed in January 2025 and published in July 2026 before issuing this week. It describes cameras trained on the Christmas Tree, software that reads the light sequence, and a control system that manages braking, torque and throttle through the start.

How the system is meant to work

According to Road & Track, the system uses onboard cameras to identify the Christmas Tree and track its light pattern in real time. Once the car is staged, it can prepare for launch before the green light appears, holding brake pressure and staging torque automatically after the driver releases the pedals. The car handles the holding and the timing rather than the driver's foot.

Road & Track described it as the possible dawn of push-button drag racing and said the patent could change how amateur racers approach a starting line, adding it would be eager to see how close the system could come to cutting a perfect light if GM ever builds it.

The automation follows a pattern at GM, which has also patented active aero control and an over-the-air camera update system, both aimed at letting software do work that used to sit entirely with hardware or driver input.

Pro Tree and Sportsman Tree timing

The patent's most specific detail is how it distinguishes two starting-light formats. A Pro Tree fires all three amber lights simultaneously, with the green appearing 0.4 seconds later, rewarding reaction time over anticipation. A Sportsman Tree fires its ambers in sequence, spaced half a second apart, before the green comes on. The system is designed to recognise which format it is looking at and adjust its predicted launch timing to match.

The two formats reward different skills, so a system that guessed wrong would either jump early or launch late. That distinction is the heart of the idea: a camera that can tell the formats apart is doing the job a driver's eyes and reflexes normally do, and doing it without the human error that decides so many amateur drag races.

What GM has not said

The patent does not specify how the camera tells the two formats apart in poor lighting or at unfamiliar tracks, details that would matter to anyone racing with it. Nothing in the filing names a vehicle, a model year or even a vague production timeline, and GM has not confirmed any plan to bring the system to market.

That leaves the obvious questions open. Which models would get it, whether it would arrive as a software feature on existing cars or only on new ones, and what it would cost are all unannounced. For now the patent is a statement of intent rather than a product, and drivers will only benefit if GM decides the system is worth putting on a showroom car.