"Hmm..."
Jin was deep in thought, considering the request from Marcia and Rodrigo.
There were several ways to increase both the RPM and the torque. First, if he were to use materials up to Hourai Island standards, he could improve performance to more than ten times its current level. However, that wasn't a practical solution for general use.
"In that case..."
In a real reciprocating engine—one that converts the back-and-forth motion of a piston into rotational motion—parameters like the bore and stroke were crucial. The bore was the diameter of the piston, and the stroke was the distance the piston traveled.
The ratio between the two was called the bore-stroke ratio, a standard indicator of an engine's characteristics. While Jin didn't know the finer details, he understood the basics: a shorter stroke allowed for higher RPM but lower torque, while a longer stroke lowered the RPM but increased the torque.
He also knew that to increase torque, one simply had to increase the number of cylinders. V8 or V12 engines were classic examples of this.
"A radial engine..." he mused. Those were common in aircraft. "But maybe an inline or V-type would be better here."
He found himself remembering the Golem Engines he had built for automobiles.
"But an inline engine would be way too long..."
"Um, Jin?"
"I'm pretty sure I heard that horizontally opposed engines cancel out each other's vibrations," he muttered.
"Hey, I'm talking to you!"
"If I make it a horizontally opposed 8-cylinder... no, an 8-linked engine..."
"Hello? Is anyone home?"
"...Marcia-san, when Jin-nii gets that look on his face, it’s best to just leave him alone for a while."
Elsa had clearly come to understand Jin's habits well.
"Alright, it's decided. Marcia, I think I can do it," Jin said, turning back to her as if nothing had happened.
"O-Oh, I see. T-That's good to hear."
"Then let's get started on a prototype right away."
But he was immediately stopped.
"Jin-nii, it's almost time for lunch."
The clock had just passed 11:00 AM.
"Father, skipping meals is not allowed," Reiko added firmly.
The time difference between Hourai Island and Port Rock Town was about two hours and forty minutes. Though they had eaten breakfast at 9:00 AM Hourai time, it was now 11:00 AM in Port Rock, which meant it was 1:40 PM back home. They were quite hungry.
Marcia and Rodrigo were aware of the time difference. "You're right, Jin. Let's break for lunch first," Marcia said.
She led them out of the workshop toward the storefront to prepare food. Since Jin had built an extension on the building, there was now a kitchen large enough to easily cook for ten people, allowing them to provide for Baccus and Jeremy as well.
"It's just something simple since it's only lunch," Marcia said as she served the food fifteen minutes later. It was a standard spread of bread, soup, salad, and fruit juice.
"This salad is delicious," Jin remarked. The vegetables seemed to be unique to Port Rock Town and suited his taste perfectly.
"You think so?"
"Wait, isn't this the scent of kryptona?" Elsa asked as she tasted her own portion.
Jin nodded as the realization hit him. "Ah, you're right. This is the flavor of mitsuba!"
Kryptona was what Jin knew as mitsuba. In Kaina Village, it was a common wild herb often eaten as a blanched side dish.
"It really is good in a salad, isn't it?"
Though they had used it as a garnish in soups, they hadn't thought to use it fresh in a salad before.
"Let's try that at home next time."
"Mm," Elsa agreed.
Once they finished their lunch—which was a bit late for Jin and Elsa but a bit early for Marcia's group—they finally set to work on the engine.
"I'd rather use iron than bronze for this," Jin decided. He wanted a more durable material to handle the expected increase in power.
"Oh, we have plenty in stock," Marcia replied. "We use it for the parts that need extra strength."
Since the reinforced Arrow was now capable of using Craft Magic at a level exceeding that of an average magi craftsman, the workshop's demand for such materials had risen.
"Let's see... Iron... Hmm, carbon steel with 0.4 percent carbon. It seems to have a bit of manganese in it as well, likely by chance. This will do perfectly."
To this, Jin added nickel and chromium to create an alloy steel known as SNC steel. It was a high-strength alloy frequently used for structural components.
Up to that point, Jin worked solo, but for the rest, he enlisted the help of Elsa and Arrow. He specifically wanted Arrow involved so she could handle the maintenance later.
"We'll use gunmetal for the bearings. Arrow, make sure you don't forget to keep these maintained."
"Yes, I understand."
If the bearings wore down, the engine would develop play and lose efficiency, making them a high priority for upkeep.
Jin created the crankshaft and the Golem arm sections, Elsa handled the exterior housing, and Reiko provided the muscle parts. The work was finished in about thirty minutes. In truth, if Jin had worked alone, it probably would have taken less than ten.
"Alright, the prototype is done. Let's give it a spin."
The prototype engine was approximately 50 centimeters long, 25 centimeters high, and 50 centimeters wide, weighing just under 30 kilograms. He used a mana tank as the energy source. Since Port Rock Town was at a lower latitude, the concentration of free ether was lower, making it difficult for anyone but Jin to operate the engine efficiently without a dedicated tank.
"Start."
The engine began to hum. Because it was a horizontally opposed design, the vibrations were minimal and the operation was quiet.
"Yeah, the RPM is up thanks to the shorter stroke."
Reiko counted the rotations of the marked shaft. "It is between 1,000 and 1,300 rotations per minute."
The torque seemed sufficient as well. It was a successful first step.
"Jin, this looks like it'll work perfectly. Thank you!"
"Jin-dono," Rodrigo added, "given the quality of this, I would like to establish a formal contract for these."
The new engine would be added to the list of components Hourai Island supplied to the Marcia Workshop. They couldn't do it for free, so after considering material costs and technical fees, they settled on a contract for five units a month—the same size as the prototype—at 20,000 Toll each. This brought the monthly total to 100,000 Toll.
Back on Hourai Island, the Smith Golems under Laojun's direction would likely handle the actual production.
"Well then, Jin, thanks again."
"No, I should be thanking you. This gave me some great ideas."
At 3:00 PM Port Rock time, Jin, Elsa, and Reiko returned to Hourai Island.
"That trip was very productive..."
『Yes, My Lord. I have been listening. The communication devices and the viewing room have already been added to the blueprints,』 Laojun reported immediately. 『However, I will have to ask My Lord to handle the design of the relay bases.』
"Right, I'll take care of it. I also want to look into the relay bases and... an entertainment room."
While interior details like that could be added later, it was better to address anything that might affect the core design as early as possible.
"Jin-nii, I'm going to consult with Mother about the meal plans."
"That would be a huge help. Thanks."
"Mm."
They wanted at least one meal a day to feel like a proper dish from the surface, which meant they had to consider everything from ingredient selection and preservation to the actual cooking methods.
Leaving that to Elsa and the others, Jin set his mind to designing new equipment, starting with the communication relays.