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Politicians and technocrats have been trying to address the impacts of human activity on the environment for decades. Particularly in the 1960s and 1970s it became painfully apparent that the air and water we rely on were tainted with the byproducts of industrialization and consumption. There was considerable improvement, particularly in air quality, over a fairly short period of time. Manufacturing, power production, packaging, and transportation were all addressed in this effort. The focus of my combustion engine design project admittedly will not have a huge impact on these issues, but it can help a bit with lowering greenhouse gas (GHG) emissions by increasing fuel efficiency while also lowering operating costs by decreasing fuel consumption. OEMs almost always work on either increasing fuel efficiency or reducing fuel consumption independently, the new Jeep Hurricane II 4-cylinder engine and Mazda’s Skyactiv engine alternate between different modes to affect each depending on load.

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There was a renewed interest in the environmental impact of human activity when scientists (and politicians) announced that not only were we affecting the environment in ways we can see, but we also affect the climate. This news was met with varying degrees of concern, disdain, and indifference. Regulatory intervention was met by resistance from vehicle manufacturers and oil producers who worried about profitability. Stricter automobile emissions rules were put in place in the 1990s and early 2000s, American and Japanese manufacturers mostly responded with improvements to engine controls and lighter structures in vehicles (while also improving safety). In the 2010s and 2020s, regulations went further and started focusing more broadly on GHG emissions while also trying to move vehicle manufacturers towards specific “prescribed” solutions. European rules went further and meant that gasoline (petrol) was frowned on in new vehicles.

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The European approach left manufacturers scrambling and a large and fast transition to a “better” fuel was implemented. Diesel motor vehicles in Europe reached almost 50% market penetration when a terrible realization came; Diesel engines burned less fuel but were polluting the environment with even more harmful toxins but manufacturers had been hiding this knowledge. At the time, Volkswagen was the world’s largest vehicle manufacturer but billions of dollars (and Euros) in penalties and the cost of reversing course piled on the losses. They had cheated on the tests and got caught. European and American regulators and politicians were appalled because they had been caught not paying attention to the technical realities and quickly passed the blame on. 

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It was critical that a new approach be put in place before people realized that they were being misled. In fact, although it is in everyone’s best interest to improve vehicle efficiency, it is not a very good solution to climate change (I’m not denying that we affect climate, just that improvements in private vehicles have reached the point of diminishing returns). There are larger problems to solve in multiple sectors of the economy, and so vehicle regulations soon doubled down and compliance with the new buzzwords “zero emissions” was enforced. EVs to the rescue, they were cheap, efficient, and nonpolluting – except none of that was true. What was important is that governments required meeting “standards” that were without technical merit and that penalties forced manufacturers to play along. Now in 2025 and 2026, manufacturers have written off billions in investments from pursuing this EV “red herring”  -- as Adrian Newey had warned in his 2017 autobiography “How to Build a Car”.

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This is not to say that EVs are not an interesting product or that you shouldn’t buy one if you want to. The facts are that governments could not justify continuing to support the purchase financially (through tax incentives), force compliance with draconian rules that were not proportional to the impact of non-compliance or prevent consumers from continuing to prefer alternatives. EVs are constructed with heavy batteries that are rich in rare-earth materials and must be replaced after a few years, generate about as much pollution as a gasoline powered vehicle over about 5 years of use if you include production related emissions, and they require generation of electricity that results in emission of GHGs and stresses an already overworked network of power grids. Add to that other factors, such as range anxiety and recent safety concerns (large EV batteries burn toxically and electric vehicles can trap unwary passengers in a crash) and it’s clear that EVs are not the future, at least for now. Again, if you want one, more power to you but don’t raise taxes so a richer consumer doesn’t pay for the extra infrastructure, electric power production capacity, and more expensive materials to support their purchase (statistically, EVs are purchased by people who “live in nice neighborhoods”).

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EVs contain a grain of genius, and it is “regenerative braking”. It’s not the “sexy” part but it recovers energy that is otherwise wasted as heat in braking and uses it productively to supplement acceleration to reduce fuel consumption (a little, they are not “magic”). Consumers have shown a preference for combining this feature with gasoline engines in hybrid vehicles, and mild hybrid vehicles are the most economical to build and least environmentally damaging of these. At the same time, combustion engines need to be as “clean” as practicable and still retain the characteristics that consumers prefer. For these reasons and more, my project entails creating a powerful family of gasoline (petrol) combustion engines and mild hybrid drivetrains that run efficiently and economically at the same time. To better absorb development costs, as many shared parts as possible will be specified across different engines and drivetrains.

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The pattern in recent years has been for manufacturers to downsize combustion engines to improve efficiency, but then they turbocharge them to put in some “pep”. This combination reduces weight (one way to reduce fuel consumption) but does little otherwise to improve efficiency (an even better way to reduce fuel consumption). When the engine is placed under load (you are not coasting), it must produce a certain amount of power and the greater the load the greater the power required but inefficiency in the engine means extra wasted energy beyond what is required by thermodynamics (which seems to limit personal transportation vehicles to about 50% peak efficiency, even if the combustion takes place in an electric power plant miles away -- as it does with EVs). In daily operation many combustion engines are only about 20% efficient, meaning fuel is consumed with 5 times the energy content that is being used productively.

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Fuel contains a set amount of power and the energy made available over time depends on how quickly you burn it, for gasoline this amounts to 44 hp-hr. per gallon (9.3 kWh per liter). This means that from one gallon of fuel you can produce 44 hp for an hour or 88 hp for ½ hour, etc. To calculate how much energy is put to work, multiply an engine’s efficiency by the energy content of fuel burned; to solve for fuel flow rate divide both sides by the efficiency, to match a 44 hp load at 30 percent efficiency you must burn at a rate a 1 gallon per hour (44 hp-hr.) divided by 30 percent or a fuel consumption rate of 3 1/3 gallons per hour. Turbocharged engines tend to produce roughly 10 hp for each pound of air consumed in a minute, although it varies with efficiency, so power is usually over-estimated for low load air movement (much of the air moves through the engine without chemically reacting, except the oxygen in the air).

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For example, if you are traveling at 60 mph (to keep the math easy) and your vehicle needs 22 hp to maintain speed and power all accessories (not far off for some vehicles) at 25 percent efficiency you need 22/ 44 = 1/2 gallon of fuel divided by 25% efficiency or 2 gallons per hour. Since you will travel 60 miles burning 2 gallons of fuel, that is 30 mpg (again, not far off for some vehicles). It is difficult to reduce the demand by ½ (although some “hyper mileage” drivers try), but doubling the operating efficiency is not as hard as manufacturers make out; it does cost money, so OEMs mostly ignore efficiency under low load conditions. Note that I am talking about doubling the operatingefficiency, not some peak number that the engine never actually reaches in daily driving (that part is what OEMs try to maximize, then they work backwards towards economy). For reference, 2 gallons of petrol is about 12.2 lbs. of fuel and since the engine is likely operating near stoichiometric (the chemically “correct” ratio of 14.7 : 1 by weight) that requires 14.7 x 12.2 = 179.4 lbs. of air per hour or almost 3 lbs. of air per minute (which we use to estimate about 10 x 3 = 30 hp – a bit high as expected).

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Doubling the available power from fuel consumed (efficiency) at highway cruise (based on CAFE standards) is a primary goal of Mutable Motors’ engine design and operating methods. In addition, plentiful but not ludicrous power potential is planned. I have owned two passenger vehicles with about 400 hp and found them enjoyable to drive; however there seems little justification for most drivers to have over 500 hp available. Most personal vehicles become traction limited at this point anyway. Since this is meant to be an international product, I set the V6 variant’s planned output at 300 kW and the V8’s at 400 kW (the higher output V10 and V12 focus on peak torque targets since maximum rpm, and thus power, will be a limited proportionately).

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Combustion engine processes are well understood and the way thermal efficiency can be improved is limited. For instance, you can increase the compression ratio (or expansion ratio if using Atkinson / Miller cyclevalve timing), although there are diminishing returns and problems that can arise include detonation (uncontrolled fuel combustion), excess heat, and increased stress on the mechanical parts. Turbocharging can effectively increase airflow (this is known as increasing volumetric efficiency), but power often has to be limited by decreasing compression, adding extra fuel to quell the heat, or retarding the spark (all of this decreases the efficiency of the engine).

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A second method to increase thermal efficiency is to improve the dispersion of fuel within the air / fuel mixture. This has improved over time as fuel injection and port fuel injection with electronic controls were adopted. Heating the fuel to cause vaporization can improve combustion speed but may reduce torque due to too-quick combustion. An effective target seems to be about 70 to 80 percent vaporization and direct injection into the cylinder absorbs excess heat during compression to affect this, port injection systems use intake track heating for vaporization, and they also help clean the back of the intake valve. Many modern systems use a combination to maximize these benefits. Runner and valve seat designs that help mix the fuel and air better (swirl and tumble) also help support complete and quick combustion.

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Cylinder offset can improve efficiency slightly, by better aligning the connecting rod with the cylinder centerline while the cylinder is at peak pressure (usually between 10 and 15 degrees after top dead center) to reduce friction between the pistons and cylinder walls. A common offset in gasoline engines is 8 mm; it is seen more often in smaller motorcycle engines and car engines emphasizing economy. My plan is to adopt the geometric equivalent of tilting the liner 3 degrees with the piston pin remaining along the same centerline as the original cylinder bank angle. Longer connecting rods also help minimize their angle to the cylinder walls, which can help a bit. Rod length to stroke ratios near 1.6 are common and the effect of small changes are minimal, our rod length specification is 152 mm, and the stroke is 77 mm, so the ratio is 1.97.

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That’s pretty much it for design level improvements, aside from addressing constraints due to material properties and friction reduction. Some operational adjustments can help as well. Chief amongst these is probably causing the throttle plate to open more. This can be affected by the design and operation, as a smaller engine has to open the throttle more to deliver the same power as a bigger engine (all other things held equal, more air equals more power). Transmission design and gearing can cause the engine to operate at a lower speed, thus requiring more air for each engine cycle and a more open throttle. Even the vehicle’s aerodynamic drag can influence how far the throttle must be open. 

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Throttle opening is directly affected by operation, the driver can refrain from opening the throttle too much for acceleration, which if more open would introduce more air which in turn results in the burning of more fuel; counterintuitively this extra is burnt more efficiently, just not economically. The driver can also close the throttle earlier to minimize braking and reduce fuel consumption (some engines are tuned to cut fuel flow under these conditions), but this is only transient. Some cylinders can be deactivated so that the air processed each engine cycle is reduced, requiring more throttle opening, it happens in operation but requires some design and component changes. This one factor has gotten more press than most others. There is a limitation to any improvement because the throttle’s effect is limited, you can’t save more than is “wasted”, and that’s around 20% with a 2-liter engine in a personal vehicle.

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Variable valve timing (VVT) has also gotten a lot of press lately. By matching the valve opening timing and duration to the air requirements of the engine at each engine speed and load condition, pumping losses can be reduced and air velocity through the runners can be optimized. The design and hardware changes allow the engine to respond better to operating changes. Again, the effects are noticeable but limited. Another method used to reduce fuel burned is to offset some air in the cylinder with inert exhaust gases. The traditional approach simply reroutes some of the exhaust to the inlet side of the engine, although valve timing can also retain some exhaust gas. The newest development in EGR involves cooling it to reduce temperatures in the combustion chamber. Finally, pressure can be relieved from the crankcase by using a one-way valve so that blow-by gases do not reduce the effectiveness of the power stroke or dilute the oil with combustion byproducts (dry sump lubrication systems use the oil pump for this pressure reduction). Each of these concepts () is implemented in the engine design proposed, except cylinder deactivation, which I view as a dead-end strategy. New devices are used to simplify the process and add fine control over the methods.

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A factor in engine thermal efficiency (as opposed to volumetric efficiency) that gets less attention than it deserves is operating speed. Drivers want high engine speed for responsiveness while fuel economy specialists want low engine speed for lower power output and to force the throttle open more. Both harm fuel efficiency; this may be the first time you’ve heard the truth, but it is a lie to equate efficiency with economy, often they are at odds. Engineers use a common metric to compare engines of different sizes and purposes. It is abbreviated BSFC, short for Brake Specific Fuel Consumption, and the units represent the quantity of fuel consumed to produce a measure of power (and power is force multiplied by speed, so the graph shows engine speed and torque). In the U.S., we tend to use pounds of fuel per horsepower hour (weight being a better measure of fuel’s heat capacity than volume because fuel expands significantly with higher temperature without gaining heat capacity – the reason you never top off a fuel tank in Alaska in the winter if you garage the vehicle) and elsewhere grams per kWh is commonly used. Under standard conditions, gasoline weighs about 6.2 lbs. per gallon (755 grams per liter, and it varies with the blend) with a heat content equivalent to roughly 44 hp-hr. per gallon (9.3 kW per liter). 

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To understand the engine’s operating efficiency, a plot is made of BFSC across engine speed and torque output. In general, the lowest BFSC occurs at low mid-range engine speeds and mid high-range torque (for instance 2400 rpm at 85% throttle might be best for one engine). At all other operating parameters, efficiency falls – importantly, economical operation requires low torque output and lower rpm (but well above idle) simply because extra power production beyond that required is wasteful, but this increases BSFC. At even lower engine speeds, the heat loss to the combustion chamber robs power and at high rpm friction heat loss is increased. At lower torque output at a given engine speed the pumping losses are increased due mainly to the throttle being closed and at higher torque output extra fuel is typically required to prevent detonation by absorbing heat. A minimum BSFC of 250 g / kWh is not uncommon but the same engine might operate closer to 400 g / kWh or more in daily driving. To convert this to vehicle fuel consumption requires you to know the power required to move and accelerate the vehicle plus accessory draw (these numbers are highly protected by manufacturers). For instance, I’ve only owned 1 car with a “torque meter” (a Kia Stinger), which could be used with fuel flow data to calculate efficiency. 

researchgate.net (see top). For reference 70 Nm is about 52 lb.-ft of torque and at 2400 rpm this equates to 24 hp; this low power engine perhaps has a 75 hp peak. A more powerful engine would follow the general trends, but BFSC would be even higher under low load, and the most efficient operation is still near 80% of peak torque.

Lean burn technologies have received a good deal of press over the past decade or so. If you run a leaner mixture, then you can configure an engine to reduce the fuel consumption for a given airflow and you can reduce emissions and increase fuel economy. There are technical challenges. First, if you run a “perfect” ratio of fuel to air (a “stoichiometric ratio” of 14.7 parts air to 1 part fuel (by weight) will burn both completely and is known as a Lambda of 1.0) there is no extra fuel to absorb heat during combustion, and you might experience detonation (that’s bad). Second, with increased heat in the combustion chamber, very toxic nitrogen oxides (NOx) are more readily produced. Interestingly, if excess air is used (a Lambda over 1.0) then temperatures decrease (as does NOx production) but the mixture does not combust as predictably if it is very lean. The solution is fairly simple and was first introduced roughly 100 years ago; a slightly richer mixture contained in a small module (called a pre-chamber) near the spark plug is used to eject fast moving flame “jets” through small holes (known as Turbulent Jet Ignition, TJI), the jets provide higher energy to ignite the overall lean mixture in the combustion chamber. This leaves us with an efficient engine that produces very low power for its size (due to the reduction of fuel mass being burned). Formula 1 engine designers found that you can increase the air mass (and thus fuel mass and power) via turbocharging and then use Atkinson / Miller cyclevalve timing to reduce the combustion pressure and increase the expansion ratio (they used to run 18 to 1 but in 2026 the rules reduced this somewhat while stirring a controversy) while increasing the engine’s reliability. When done properly, this can result in over 40% thermal efficiency and high power, although the engine does not run well below 4000 rpm or under low loads.

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To get to 50% efficiency, Formula 1 used motor / generators linked to the turbochargers to harvest waste heat energy and hybrid motor-generators for regenerative braking. There is not much progress expected above 50% efficiency, due to material, fuel, and hybrid system limits. The modern production engine record seems to be about 40% efficient, although that engine (the Jeep Hurricane II 2-liter 4-cylinder gasoline engine) doesn’t always use this mode in daily driving and reverts to a traditional spark ignition setup under low loads, so the overall impact is limited.

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The final method and devices proposed for this trial I call “Impulse Modulation Control”. My implementation is based on electronically controlled pneumatic valve “dual acting” actuators (this is different from the Formula 1 pneumatic valve springs of old, and in construction and operation does not share much with FreeValve’sversion). I will not be giving details of this proprietary approach here; although I was awarded a utility patent in 2015 by the USPTO on the pneumatic supply system I am holding the main ideas closely for now. The goal is to reduce the disparity between peak and daily driving efficiency in road going petrol combustion engines, reduce GHG emissions, and produce good power upon demand. Any good engine designer can use modern tools to design a gasoline engine that will deliver a peak efficiency of 40 percent or higher, the highest being roughly 50 percent. The problem is that in daily operation they will fall well short of this and that is the issue I am focused on, reducing the efficiency drop when the engine is under low load.

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My proof of design vehicle, a C4 Corvette with a bespoke 3.6-liter V8 engine and proprietary 9-speed transaxle, has the following targets: 400 kW maximum power at the crankshaft (over 500 hp), able to drive a standard fuel economy measurement “street course” while achieving an average at least 45% thermal efficiency. In daily driving the goal is 4 liters per 100 km, roughly 57 mpg while meeting most carbon and nitrogen oxide emissions standards (CARB rules are designed to phase out combustion engines and probably can’t be met), and on a closed oval track complete a 100-mile high speed run in 1 hour consuming only 5 gallons of fuel or less.

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Incorporating all the information presented above and addressing power production under different loads, the Mutable Motors design parameters include the following common parts and specifications: cylinder bores 87 mm in diameters (using Darton MID sleeves for Honda K20 or similar size, this sleeve supports an 87mm diameter bore with 132 mm length) and a 77 mm stroke, while the other crankshaft specifications allow for good bearing size (likely Honda K24 bearing width specs, given the planned cylinder bore spacing of 94 mm), with a reluctor wheel and sensor crank timing signal. The crankshaft will be a cross-plane type with GM pin arrangements and cylinder numbering for the V8, other crankshafts will include so-called flying pins if needed for even fire operation. Piston and combustion chamber designs includes 16 to 1 compression ratio, two intake valves of different sizes (24 mm and 35 mm diameter), one 31 mm exhaust valve, a proprietary “semi-passive” pre-chamber for TJI mounted so it covers the slightly recessed central spark plug, direct fuel injection focused on enriching the region around the pre-chamber with port injection providing an overall higher Lambda mixture, thin ring package with gapless second ring, nickel coated crown and polymer coated skirts, 25 mm pin compression height. Connecting rods have 22 mm wrist-pin bores and are 152 mm center to center with 51 mm big end diameter, 19.8 mm thick (based on Honda K24 specs, may be slightly different spec depending on availability). The block design includes a Compressed Graphite Iron (CGI) mid-block with cracked main bearings and aluminum girdle and heads (similar to Ford’s 2.7-liter EcoBoost V6 construction).

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Cylinder heads will include 3 valves, seats, and runners for each chamber and they will be arrayed so that the small intake valve is mounted along a line parallel to the crankshaft center line passing through the spark plug hole; the other valves will be allocated space as practicable so that the single exhaust runner exits the head inside the V shape of the engine (hot V). Exhaust manifolds will include equal length runners with “cross overs” for cylinders 3,4,5, and 6 to equalize pulse spacing. Intake plenums fitted outside the engine V will feed pairs of intake manifolds tuned for low engine speed (small valve) and medium engine speed (large valve) operation. Each valve will be equipped with the usual seat and stem fittings. Along the end of the stem, each valve will be equipped with a pneumatic actuator, approx. spec. 50 mm inside diameter, the actuators piston will translate 12 mm via dual pneumatic actuation and the actuator will attach to the valve via a combination “keeper” style piston attached with a screw cap fitted so as to draw the valve stem end and keeper together for positive retention through the valve actuation cycle. A mechanical spring with 60 lbs. of seat force will act to supplement the pneumatic piston to close the valve. The manifolds supplying the chambers of the valve actuators will be electronically pressure controlled so as to vary the force on the valves with engine speed, thus correlating force applied to engine speed. Proprietary pneumatic gates are used to provide the required fast cycling of the actuating chamber pressure acting to open the valves while the actuating chamber acting to close the valve acts as a combination mechanical and pneumatic spring with check valves serving to regulate pressure. This is explained in more detail in a separate article.

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Water and oil pumps will be electrically actuated, and the engine will have a dry sump with a multiport external pump and remote tank. There will be no alternator, the mild hybrid ISG and motor-generators will recharge the electrical system batteries. The number of motor-generators will vary with vehicle model but will always include an eTurbo unit and an ISG coupled to the transaxle, some vehicles will have “helper” motor-generators for the front wheels.

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A pair of sequential, compound turbochargers will provide compressed air to the intake manifolds (mounted outside the “V”) after passing through air-to-air intercoolers. The smaller turbocharger (which I will label “primary”, although there is not agreement on labels within the industry) will function as a stand-alone turbocharger under low loads and will incorporate a motor-generator to supply the 48-volt electrical bus during low load operation. The primary turbo’s compressor wheel is sized for 5 to 20 lbs. of airflow per minute and designed to provide a boost pressure for a small engine, one candidate is the Garrett GBC14-250. The secondary turbo’s compressor is sized to provide 15 to 50 lbs. of airflow per minute and might be a Garrett G30-770 or Garrett GTX3076R GEN II operated on the low end of its compressor map. Together, these turbos will provide proper airflow for efficient operation under a wide range of loads and engine speeds.

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The engine and 9-speed transaxle will connect through a drive mechanism; the preferred embodiment is a 3: 1 planetary gear reduction system mounted on a flywheel and a drive shaft fitted with constant velocity joints. The transaxle will contain an input shaft and an output shaft each fitted with 3 gears engaged selectively via hydraulic “wet clutch” mechanisms controlled with hydraulic mechatronic circuits. An intermediate shaft will transfer motion between active gears on these two shafts via three fixed gears such that there is a three-by-three array of gears along the shafts thus allowing selection of nine distinct gear ratios. Gear ratios are designed to provide an engine speed reduction from 2700 rpm to 2100 rpm with each shift when operated in “economy mode”, note that this requires conical gears and angled shafts. The intermediate shaft will also connect an electric 48-volt ISG (housed within or affixed to the casing) which will also function as the vehicle’s reverse gear. When the driver wishes to shift gears manually or launch from a standing start, paddle shifters and a hand clutch will be in place for ease of use.

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In his seminal 2017 book “How to Build a Car”, Adrian Newey summed up his efforts by lamenting that his work had yet to influence road cars, particularly as it pertains to cars most people can afford to drive. He said that reducing the carbon footprint of interesting, affordable cars would satisfy a life-long ambition. This parallels my own design efforts, however without the rich experiential background of designing some of the world's most powerful, efficient and interesting racing cars and sports cars. My focus has also been more on engines and drivetrains than aerodynamics, although I am aware of and interested in the aerodynamics of passenger vehicles.

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There really is not much that hasn’t already been explored when it comes to making combustion engines more efficient and less polluting. That is not the same as saying everything has been tried. I think I have found a niche to exploit, and I think times demand this solution -- a family of powerful yet economical engines that can be applied to a wide range of products. My current plan is to construct and test a purpose-built 3.6-liter V8 using everything practicable to improve efficiency, economy, and power within the power-plant and drivetrain. My proof-of-concept vehicle is a C4 Chevrolet Corvette, which I already own, although if Mr. Newey wishes to design a “saloon car” to fit, that would be the ultimate. Although this is very much meant as a technology demonstrator, an eye is always cast toward a marketable product.

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To that end, I have stressed parts commonality across a family of engines to reduce development cost and time, simplify parts supply chains, and make maintenance skills easy to apply to a number of vehicles. The goals include manufacturability, operating, and design simplification in what is by necessity a complex machine. It is expected that vehicles using these engines and drivetrains will also implement these strategies. At the same time, plenty of power is available on demand to complement an interesting and fun vehicle. The vehicles should combine safety, comfort, and handling while also keeping controls intuitive and minimize distracting from the task of driving. Any “glass” displays should be for supplemental tasks (like navigation and backup cameras). Drivers should not be artificially separated from the task at hand unless for a mission priority (such as economy, safety, and durability).