A Brief History Of Combustion, Part 3

Suzuki GSX-R1000: “Today nearly all auto and motorcycle four-stroke engines base their cylinder-head designs on that of Keith Duckworth’s DFV, with four valves, efficient downdraft intakes, an essentially flat-topped piston, with a single central spark plug.”

A combination of axial-charge swirl generated by an offset intake port and squish action between piston and head surfaces was effective in making combustion faster and more efficient in the 1950s (in this context, “efficient” means a high percentage of the charge is burned and heat loss is kept to a minimum by a speedy burn). The big single-cylinder motorcycles in which these discoveries had been made had run their course, but with money to spend, Formula 1 continued to seek higher power from more revs.

The problem with higher revs is that your engine runs out of breath as its airflow needs begin to exceed the capacity of its valves. BRM in England had at first tried increasing valve area by using a great many cylinders in the 1940’s V-16, but that was too complex for its R&D budget. Designer Stu Tresilian then switched to a simpler four-cylinder design, the P25 with a bore much larger than its stroke to make room for two very large valves per cylinder. Controlling the motions of these big “hatches” was non-trivial, with many valve/piston collisions resulting.

Mercedes sought to solve this problem with more cylinders and direct valve control. Its “Z-Drive” desmodromic system opened each valve with one cam and closed it by another. The complex all-roller, two-valve, eight-cylinder W196 GP car won championships in 1954-55, but the desmo idea did not catch on.

A fourth concept already had a long history: increasing valve area by using four instead of two per cylinder. Beginning in 1959, Honda explored the use of four valves by building and comparing two types of 125cc four-stroke twins, one with two valves per cylinder, the other with four. The two-valve engine seemed to breathe or burn a bit better but the four-valve had no valve-control issues. This was in a time when the acknowledged “masters of the two-valve art,” Italy’s MV Agusta, had a bare 300 revs between peak power and valve toss (in those days, the rider was the only rev limiter). Using the superior control of four valves, Honda was able to push rpm and power up quickly, despite its engines’ slow combustion.

In 1964 at F-1 engine-builder Coventry-Climax, engineer Walter Hassan decided to move power up in rpm by putting four-valve heads on the 1.5-liter V-8, but it made poor power until he realized it needed 10 degrees earlier ignition timing (Honda’s four-valve race engines of the 1960s needed even more). Hassan attributed this slowed combustion to two causes: 1) that axial swirl was less workable with twin intakes; and 2) that he thought the valve angle chosen was too wide, requiring an intruding piston dome that damped out turbulence. Squish alone was unable to compensate for this.

By the middle 1960s, these problems were a log jam in four-stroke engine development. As before, lots of small, relatively “square” cylinders, with high revs made possible by their short strokes, seemed to offer a way out. Honda built tiny, watch-like fours, fives, and sixes to power its Grand Prix bikes, and F-1 builders went to 12 and even 16 cylinders. It was an era of overwhelming basic problems with more revs.

A better way was in gestation, as Englishman Keith Duckworth sought to improve combustion in his radically oversquare (bore larger than stroke) “SCA” F-2 engine. He had learned to get a lot of air into this engine by use of a steeply downdraft yet quite small intake port but somehow could not achieve rapid combustion. Duckworth did not accept “official right answers,” instead insisting upon seeking his own answers directly from reality.

Yamaha YZF-R1: “Duckworth saw that fast intake flow from his two downdraft ports would flow into the cylinder, hit the far cylinder wall, flow down it to the flat piston crown, cross it, then climb the near cylinder wall in a looping path.”

Despite this, three years of effort did not get the SCA’s ignition timing to values less than 49 degrees BTDC, indicating dismally long combustion with high heat loss. Like Hassan, Duckworth had tried to use squish alone to generate charge turbulence that would accelerate combustion, but despite many experiments, it could not be made to work. He needed a new idea.

Given the opportunity to design a new F-2 engine, Duckworth was attracted to four valves and looked first at Ludwig Apfelbeck’s radial four-valve. By having its two intakes at opposite sides of the cylinder, its twin intake ports could be offset to produce traditional axial swirl. Duckworth rejected this for its complexity but continued thinking about four valves.

Race engine designers of the mid-1920s had placed their two valves at a large included angle of 90-100 degrees because that gave room for bigger valves. From the 1930s to the ’50s, improved port flow and higher valve lifts had allowed this to be reduced to 60 degrees, producing a shallower, faster-burning chamber (a good example is the chambers of 1960’s Norton twins).

Duckworth’s SCA experiments showed he could get plenty of flow through even smaller valves. This told him there was no longer any reason to use large valve included angles, so in his next design, he bypassed the 60 degrees then considered “right” and adopted 40, allowing him to almost entirely dispense with any piston dome.

Now Duckworth saw that fast intake flow from his two downdraft ports would flow into the cylinder, hit the far cylinder wall, flow down it to the flat piston crown, cross it, then climb the near cylinder wall in a looping path. As the essentially flat-topped piston rose on compression, there was no dome to cause this motion to die out, allowing the looping motion to remain energetic enough to speed combustion.

When this concept was refined (Duckworth called it “barrel motion,” later renamed “tumble”), it enabled his new 1600cc four-valve “FVA” engine to make 200 horsepower in March, 1966, on very reduced ignition timing. Duckworth’s “barrel motion” stored high intake velocity in its looping path, becoming vigorous turbulence near TDC that resulted in fast, low-heat-loss combustion.

Design of the FVA was part of a Ford-funded package that included an F-1 engine, as well. Duckworth liked simplicity, so he chose to build a V-8 in a time when the known “right answer” was a V-12. Using his FVA experience, he designed the DFV F-1 engine with an even smaller valve included angle of 32 degrees. It won the first race in which it was entered—the first of 155 F-1 victories. This engine required a peak-torque ignition timing of only 27 degrees BTDC, indicating fast combustion, indeed.

Matra, the French aerospace giant then participating in F-1, fielded a conventional four-valve V-12 that was expected to dominate because of its overwhelming total valve area (always a great favorite of armchair analysts). Its traditional wide valve angle, deep combustion chambers, intrusive piston domes, and slow combustion allowed Duckworth’s open-chamber V-8 to leave it for dead. All competitors had to understand and adopt Duckworth’s ideas in order to have any chance of success.

Italian engineering student Massimo Bordi went to Cosworth to study Duckworth’s work, then returned to Ducati to eventually design the “Ottovalvole” (eight valve) from which all subsequent Ducati Superbike engines have evolved. In doing so, he had to overcome formidable resistance to the four-valve idea from Ducati’s grand old man of design, Fabio Taglioni. Early Japanese interpretations had Duckworth’s downdraft intake ports but fell far short of his fast combustion; full understanding would take time.

Today nearly all auto and motorcycle four-stroke engines base their cylinder-head designs on that of Keith Duckworth’s DFV, with four valves, efficient downdraft intakes, an essentially flat-topped piston, with a single central spark plug. Duckworth wondered out loud how so many designers could work so hard stuffing their engines with fresh mixture and then devote so little effort to burning that mixture quickly and completely. To this day, his concept of tumble motion provides a workable answer to that second problem.

Suzuki GSX-R1000: “Today nearly all auto and motorcycle four-stroke engines base their cylinder-head designs on that of Keith Duckworth’s DFV, with four valves, efficient downdraft intakes, an essentially flat-topped piston, with a single central spark plug.”

A combination of axial-charge swirl generated by an offset intake port and squish action between piston and head surfaces was effective in making combustion faster and more efficient in the 1950s (in this context, “efficient” means a high percentage of the charge is burned and heat loss is kept to a minimum by a speedy burn). The big single-cylinder motorcycles in which these discoveries had been made had run their course, but with money to spend, Formula 1 continued to seek higher power from more revs.

The problem with higher revs is that your engine runs out of breath as its airflow needs begin to exceed the capacity of its valves. BRM in England had at first tried increasing valve area by using a great many cylinders in the 1940’s V-16, but that was too complex for its R&D budget. Designer Stu Tresilian then switched to a simpler four-cylinder design, the P25 with a bore much larger than its stroke to make room for two very large valves per cylinder. Controlling the motions of these big “hatches” was non-trivial, with many valve/piston collisions resulting.

Mercedes sought to solve this problem with more cylinders and direct valve control. Its “Z-Drive” desmodromic system opened each valve with one cam and closed it by another. The complex all-roller, two-valve, eight-cylinder W196 GP car won championships in 1954-55, but the desmo idea did not catch on.

A fourth concept already had a long history: increasing valve area by using four instead of two per cylinder. Beginning in 1959, Honda explored the use of four valves by building and comparing two types of 125cc four-stroke twins, one with two valves per cylinder, the other with four. The two-valve engine seemed to breathe or burn a bit better but the four-valve had no valve-control issues. This was in a time when the acknowledged “masters of the two-valve art,” Italy’s MV Agusta, had a bare 300 revs between peak power and valve toss (in those days, the rider was the only rev limiter). Using the superior control of four valves, Honda was able to push rpm and power up quickly, despite its engines’ slow combustion.

In 1964 at F-1 engine-builder Coventry-Climax, engineer Walter Hassan decided to move power up in rpm by putting four-valve heads on the 1.5-liter V-8, but it made poor power until he realized it needed 10 degrees earlier ignition timing (Honda’s four-valve race engines of the 1960s needed even more). Hassan attributed this slowed combustion to two causes: 1) that axial swirl was less workable with twin intakes; and 2) that he thought the valve angle chosen was too wide, requiring an intruding piston dome that damped out turbulence. Squish alone was unable to compensate for this.

By the middle 1960s, these problems were a log jam in four-stroke engine development. As before, lots of small, relatively “square” cylinders, with high revs made possible by their short strokes, seemed to offer a way out. Honda built tiny, watch-like fours, fives, and sixes to power its Grand Prix bikes, and F-1 builders went to 12 and even 16 cylinders. It was an era of overwhelming basic problems with more revs.

A better way was in gestation, as Englishman Keith Duckworth sought to improve combustion in his radically oversquare (bore larger than stroke) “SCA” F-2 engine. He had learned to get a lot of air into this engine by use of a steeply downdraft yet quite small intake port but somehow could not achieve rapid combustion. Duckworth did not accept “official right answers,” instead insisting upon seeking his own answers directly from reality.

Yamaha YZF-R1: “Duckworth saw that fast intake flow from his two downdraft ports would flow into the cylinder, hit the far cylinder wall, flow down it to the flat piston crown, cross it, then climb the near cylinder wall in a looping path.”

Despite this, three years of effort did not get the SCA’s ignition timing to values less than 49 degrees BTDC, indicating dismally long combustion with high heat loss. Like Hassan, Duckworth had tried to use squish alone to generate charge turbulence that would accelerate combustion, but despite many experiments, it could not be made to work. He needed a new idea.

Given the opportunity to design a new F-2 engine, Duckworth was attracted to four valves and looked first at Ludwig Apfelbeck’s radial four-valve. By having its two intakes at opposite sides of the cylinder, its twin intake ports could be offset to produce traditional axial swirl. Duckworth rejected this for its complexity but continued thinking about four valves.

Race engine designers of the mid-1920s had placed their two valves at a large included angle of 90-100 degrees because that gave room for bigger valves. From the 1930s to the ’50s, improved port flow and higher valve lifts had allowed this to be reduced to 60 degrees, producing a shallower, faster-burning chamber (a good example is the chambers of 1960’s Norton twins).

Duckworth’s SCA experiments showed he could get plenty of flow through even smaller valves. This told him there was no longer any reason to use large valve included angles, so in his next design, he bypassed the 60 degrees then considered “right” and adopted 40, allowing him to almost entirely dispense with any piston dome.

Now Duckworth saw that fast intake flow from his two downdraft ports would flow into the cylinder, hit the far cylinder wall, flow down it to the flat piston crown, cross it, then climb the near cylinder wall in a looping path. As the essentially flat-topped piston rose on compression, there was no dome to cause this motion to die out, allowing the looping motion to remain energetic enough to speed combustion.

When this concept was refined (Duckworth called it “barrel motion,” later renamed “tumble”), it enabled his new 1600cc four-valve “FVA” engine to make 200 horsepower in March, 1966, on very reduced ignition timing. Duckworth’s “barrel motion” stored high intake velocity in its looping path, becoming vigorous turbulence near TDC that resulted in fast, low-heat-loss combustion.

Design of the FVA was part of a Ford-funded package that included an F-1 engine, as well. Duckworth liked simplicity, so he chose to build a V-8 in a time when the known “right answer” was a V-12. Using his FVA experience, he designed the DFV F-1 engine with an even smaller valve included angle of 32 degrees. It won the first race in which it was entered—the first of 155 F-1 victories. This engine required a peak-torque ignition timing of only 27 degrees BTDC, indicating fast combustion, indeed.

Matra, the French aerospace giant then participating in F-1, fielded a conventional four-valve V-12 that was expected to dominate because of its overwhelming total valve area (always a great favorite of armchair analysts). Its traditional wide valve angle, deep combustion chambers, intrusive piston domes, and slow combustion allowed Duckworth’s open-chamber V-8 to leave it for dead. All competitors had to understand and adopt Duckworth’s ideas in order to have any chance of success.

Italian engineering student Massimo Bordi went to Cosworth to study Duckworth’s work, then returned to Ducati to eventually design the “Ottovalvole” (eight valve) from which all subsequent Ducati Superbike engines have evolved. In doing so, he had to overcome formidable resistance to the four-valve idea from Ducati’s grand old man of design, Fabio Taglioni. Early Japanese interpretations had Duckworth’s downdraft intake ports but fell far short of his fast combustion; full understanding would take time.

Today nearly all auto and motorcycle four-stroke engines base their cylinder-head designs on that of Keith Duckworth’s DFV, with four valves, efficient downdraft intakes, an essentially flat-topped piston, with a single central spark plug. Duckworth wondered out loud how so many designers could work so hard stuffing their engines with fresh mixture and then devote so little effort to burning that mixture quickly and completely. To this day, his concept of tumble motion provides a workable answer to that second problem.

Loading...